feat: expense settlement — track who paid, show who owes whom — closes #41

- Click member avatars on budget items to mark who paid (green = paid)
- Multiple green chips = those people split the payment equally
- Settlement dropdown in the total budget card shows optimized payment
  flows (who owes whom how much) and net balances per person
- Info tooltip explains how the feature works
- New server endpoint GET /budget/settlement calculates net balances
  and minimized payment flows using a greedy algorithm
- Merged category legend: amount + percentage in one row
- i18n keys added for DE and EN

client/src/api/client.ts

@@ -184,6 +184,7 @@ export const budgetApi = {
   setMembers: (tripId: number | string, id: number, userIds: number[]) => apiClient.put(`/trips/${tripId}/budget/${id}/members`, { user_ids: userIds }).then(r => r.data),
   togglePaid: (tripId: number | string, id: number, userId: number, paid: boolean) => apiClient.put(`/trips/${tripId}/budget/${id}/members/${userId}/paid`, { paid }).then(r => r.data),
   perPersonSummary: (tripId: number | string) => apiClient.get(`/trips/${tripId}/budget/summary/per-person`).then(r => r.data),
+  settlement: (tripId: number | string) => apiClient.get(`/trips/${tripId}/budget/settlement`).then(r => r.data),
 }
 
 export const filesApi = {

client/src/components/Budget/BudgetPanel.tsx

@@ -3,7 +3,7 @@ import { useState, useEffect, useRef, useMemo, useCallback } from 'react'
 import DOM from 'react-dom'
 import { useTripStore } from '../../store/tripStore'
 import { useTranslation } from '../../i18n'
-import { Plus, Trash2, Calculator, Wallet, Pencil, Users, Check } from 'lucide-react'
+import { Plus, Trash2, Calculator, Wallet, Pencil, Users, Check, Info, ChevronDown, ChevronRight } from 'lucide-react'
 import CustomSelect from '../shared/CustomSelect'
 import { budgetApi } from '../../api/client'
 import type { BudgetItem, BudgetMember } from '../../types'
@@ -145,9 +145,11 @@ interface ChipWithTooltipProps {
   label: string
   avatarUrl: string | null
   size?: number
+  paid?: boolean
+  onClick?: () => void
 }
 
-function ChipWithTooltip({ label, avatarUrl, size = 20 }: ChipWithTooltipProps) {
+function ChipWithTooltip({ label, avatarUrl, size = 20, paid, onClick }: ChipWithTooltipProps) {
   const [hover, setHover] = useState(false)
   const [pos, setPos] = useState({ top: 0, left: 0 })
   const ref = useRef(null)
@@ -160,13 +162,19 @@ function ChipWithTooltip({ label, avatarUrl, size = 20 }: ChipWithTooltipProps)
     setHover(true)
   }
 
+  const borderColor = paid ? '#22c55e' : 'var(--border-primary)'
+  const bg = paid ? 'rgba(34,197,94,0.15)' : 'var(--bg-tertiary)'
+
   return (
     <>
       <div ref={ref} onMouseEnter={onEnter} onMouseLeave={() => setHover(false)}
+        onClick={onClick}
         style={{
-          width: size, height: size, borderRadius: '50%', border: '1.5px solid var(--border-primary)',
-          background: 'var(--bg-tertiary)', display: 'flex', alignItems: 'center', justifyContent: 'center',
-          fontSize: size * 0.4, fontWeight: 700, color: 'var(--text-muted)', overflow: 'hidden', flexShrink: 0,
+          width: size, height: size, borderRadius: '50%', border: `2px solid ${borderColor}`,
+          background: bg, display: 'flex', alignItems: 'center', justifyContent: 'center',
+          fontSize: size * 0.4, fontWeight: 700, color: paid ? '#16a34a' : 'var(--text-muted)',
+          overflow: 'hidden', flexShrink: 0, cursor: onClick ? 'pointer' : 'default',
+          transition: 'border-color 0.15s, background 0.15s',
         }}>
         {avatarUrl
           ? <img src={avatarUrl} style={{ width: '100%', height: '100%', objectFit: 'cover' }} />
@@ -177,11 +185,19 @@ function ChipWithTooltip({ label, avatarUrl, size = 20 }: ChipWithTooltipProps)
         <div style={{
           position: 'fixed', top: pos.top, left: pos.left, transform: 'translate(-50%, -100%)',
           pointerEvents: 'none', zIndex: 10000, whiteSpace: 'nowrap',
+          display: 'flex', alignItems: 'center', gap: 5,
           background: 'var(--bg-card, white)', color: 'var(--text-primary, #111827)',
           fontSize: 11, fontWeight: 500, padding: '5px 10px', borderRadius: 8,
           boxShadow: '0 4px 12px rgba(0,0,0,0.15)', border: '1px solid var(--border-faint, #e5e7eb)',
         }}>
           {label}
+          {paid && (
+            <span style={{
+              fontSize: 9, fontWeight: 700, padding: '1px 5px', borderRadius: 4,
+              background: 'rgba(34,197,94,0.15)', color: '#16a34a',
+              textTransform: 'uppercase', letterSpacing: '0.03em',
+            }}>Paid</span>
+          )}
         </div>,
         document.body
       )}
@@ -194,10 +210,11 @@ interface BudgetMemberChipsProps {
   members?: BudgetMember[]
   tripMembers?: TripMember[]
   onSetMembers: (memberIds: number[]) => void
+  onTogglePaid?: (userId: number, paid: boolean) => void
   compact?: boolean
 }
 
-function BudgetMemberChips({ members = [], tripMembers = [], onSetMembers, compact = true }: BudgetMemberChipsProps) {
+function BudgetMemberChips({ members = [], tripMembers = [], onSetMembers, onTogglePaid, compact = true }: BudgetMemberChipsProps) {
   const chipSize = compact ? 20 : 30
   const btnSize = compact ? 18 : 28
   const iconSize = compact ? (members.length > 0 ? 8 : 9) : (members.length > 0 ? 12 : 14)
@@ -237,7 +254,10 @@ function BudgetMemberChips({ members = [], tripMembers = [], onSetMembers, compa
   return (
     <div style={{ display: 'flex', alignItems: 'center', justifyContent: 'center', gap: 2, flexWrap: 'wrap' }}>
       {members.map(m => (
-        <ChipWithTooltip key={m.user_id} label={m.username} avatarUrl={m.avatar_url} size={chipSize} />
+        <ChipWithTooltip key={m.user_id} label={m.username} avatarUrl={m.avatar_url} size={chipSize}
+          paid={!!m.paid}
+          onClick={onTogglePaid ? () => onTogglePaid(m.user_id, !m.paid) : undefined}
+        />
       ))}
       <button ref={btnRef} onClick={openDropdown}
         style={{
@@ -376,15 +396,23 @@ interface BudgetPanelProps {
 }
 
 export default function BudgetPanel({ tripId, tripMembers = [] }: BudgetPanelProps) {
-  const { trip, budgetItems, addBudgetItem, updateBudgetItem, deleteBudgetItem, loadBudgetItems, updateTrip, setBudgetItemMembers } = useTripStore()
+  const { trip, budgetItems, addBudgetItem, updateBudgetItem, deleteBudgetItem, loadBudgetItems, updateTrip, setBudgetItemMembers, toggleBudgetMemberPaid } = useTripStore()
   const { t, locale } = useTranslation()
   const [newCategoryName, setNewCategoryName] = useState('')
   const [editingCat, setEditingCat] = useState(null) // { name, value }
+  const [settlement, setSettlement] = useState<{ balances: any[]; flows: any[] } | null>(null)
+  const [settlementOpen, setSettlementOpen] = useState(false)
   const currency = trip?.currency || 'EUR'
 
   const fmt = (v, cur) => fmtNum(v, locale, cur)
   const hasMultipleMembers = tripMembers.length > 1
 
+  // Load settlement data whenever budget items change
+  useEffect(() => {
+    if (!hasMultipleMembers) return
+    budgetApi.settlement(tripId).then(setSettlement).catch(() => {})
+  }, [tripId, budgetItems, hasMultipleMembers])
+
   const setCurrency = (cur) => {
     if (tripId) updateTrip(tripId, { currency: cur })
   }
@@ -539,6 +567,7 @@ export default function BudgetPanel({ tripId, tripMembers = [] }: BudgetPanelPro
                                     members={item.members || []}
                                     tripMembers={tripMembers}
                                     onSetMembers={(userIds) => setBudgetItemMembers(tripId, item.id, userIds)}
+                                    onTogglePaid={(userId, paid) => toggleBudgetMemberPaid(tripId, item.id, userId, paid)}
                                     compact={false}
                                   />
                                 </div>
@@ -553,6 +582,7 @@ export default function BudgetPanel({ tripId, tripMembers = [] }: BudgetPanelPro
                                   members={item.members || []}
                                   tripMembers={tripMembers}
                                   onSetMembers={(userIds) => setBudgetItemMembers(tripId, item.id, userIds)}
+                                  onTogglePaid={(userId, paid) => toggleBudgetMemberPaid(tripId, item.id, userId, paid)}
                                 />
                               ) : (
                                 <InlineEditCell value={item.persons} type="number" decimals={0} onSave={v => handleUpdateField(item.id, 'persons', v != null ? parseInt(v) || null : null)} style={{ textAlign: 'center' }} placeholder="-" locale={locale} editTooltip={t('budget.editTooltip')} />
@@ -628,6 +658,91 @@ export default function BudgetPanel({ tripId, tripMembers = [] }: BudgetPanelPro
             {hasMultipleMembers && (budgetItems || []).some(i => i.members?.length > 0) && (
               <PerPersonInline tripId={tripId} budgetItems={budgetItems} currency={currency} locale={locale} />
             )}
+
+            {/* Settlement dropdown inside the total card */}
+            {hasMultipleMembers && settlement && settlement.flows.length > 0 && (
+              <div style={{ marginTop: 16, borderTop: '1px solid rgba(255,255,255,0.1)', paddingTop: 12 }}>
+                <button onClick={() => setSettlementOpen(v => !v)} style={{
+                  display: 'flex', alignItems: 'center', gap: 6, width: '100%',
+                  background: 'none', border: 'none', cursor: 'pointer', padding: 0, fontFamily: 'inherit',
+                  color: 'rgba(255,255,255,0.6)', fontSize: 11, fontWeight: 600, letterSpacing: 0.5,
+                }}>
+                  {settlementOpen ? <ChevronDown size={13} /> : <ChevronRight size={13} />}
+                  {t('budget.settlement')}
+                  <span style={{ position: 'relative', display: 'inline-flex', marginLeft: 2 }}>
+                    <span style={{ display: 'flex', cursor: 'help' }}
+                      onMouseEnter={e => { const tip = e.currentTarget.nextElementSibling as HTMLElement; if (tip) tip.style.display = 'block' }}
+                      onMouseLeave={e => { const tip = e.currentTarget.nextElementSibling as HTMLElement; if (tip) tip.style.display = 'none' }}
+                      onClick={e => e.stopPropagation()}
+                    >
+                      <Info size={11} strokeWidth={2} />
+                    </span>
+                    <div style={{
+                      display: 'none', position: 'absolute', top: '100%', left: '50%', transform: 'translateX(-50%)',
+                      marginTop: 6, width: 220, padding: '10px 12px', borderRadius: 10, zIndex: 100,
+                      background: 'var(--bg-card)', border: '1px solid var(--border-faint)',
+                      boxShadow: '0 4px 16px rgba(0,0,0,0.12)',
+                      fontSize: 11, fontWeight: 400, color: 'var(--text-secondary)', lineHeight: 1.5, textAlign: 'left',
+                    }}>
+                      {t('budget.settlementInfo')}
+                    </div>
+                  </span>
+                </button>
+
+                {settlementOpen && (
+                  <div style={{ marginTop: 10, display: 'flex', flexDirection: 'column', gap: 8 }}>
+                    {settlement.flows.map((flow, i) => (
+                      <div key={i} style={{
+                        display: 'flex', alignItems: 'center', justifyContent: 'center', gap: 10,
+                        padding: '8px 10px', borderRadius: 10,
+                        background: 'rgba(255,255,255,0.06)',
+                      }}>
+                        <ChipWithTooltip label={flow.from.username} avatarUrl={flow.from.avatar_url} size={28} />
+                        <div style={{ display: 'flex', alignItems: 'center', gap: 4 }}>
+                          <span style={{ fontSize: 11, color: 'rgba(255,255,255,0.3)' }}>→</span>
+                          <span style={{ fontSize: 12, fontWeight: 700, color: '#f87171', whiteSpace: 'nowrap' }}>
+                            {fmt(flow.amount, currency)}
+                          </span>
+                          <span style={{ fontSize: 11, color: 'rgba(255,255,255,0.3)' }}>→</span>
+                        </div>
+                        <ChipWithTooltip label={flow.to.username} avatarUrl={flow.to.avatar_url} size={28} />
+                      </div>
+                    ))}
+
+                    {settlement.balances.filter(b => Math.abs(b.balance) > 0.01).length > 0 && (
+                      <div style={{ marginTop: 4, borderTop: '1px solid rgba(255,255,255,0.08)', paddingTop: 8 }}>
+                        <div style={{ fontSize: 9, fontWeight: 600, color: 'rgba(255,255,255,0.35)', textTransform: 'uppercase', letterSpacing: '0.05em', marginBottom: 6 }}>
+                          {t('budget.netBalances')}
+                        </div>
+                        {settlement.balances.filter(b => Math.abs(b.balance) > 0.01).map(b => (
+                          <div key={b.user_id} style={{ display: 'flex', alignItems: 'center', gap: 6, padding: '2px 0' }}>
+                            <div style={{
+                              width: 20, height: 20, borderRadius: '50%', flexShrink: 0,
+                              background: 'rgba(255,255,255,0.1)', display: 'flex', alignItems: 'center', justifyContent: 'center',
+                              fontSize: 8, fontWeight: 700, color: 'rgba(255,255,255,0.6)', overflow: 'hidden',
+                            }}>
+                              {b.avatar_url
+                                ? <img src={b.avatar_url} style={{ width: '100%', height: '100%', objectFit: 'cover' }} />
+                                : b.username?.[0]?.toUpperCase()
+                              }
+                            </div>
+                            <span style={{ flex: 1, fontSize: 11, color: 'rgba(255,255,255,0.6)', fontWeight: 500, overflow: 'hidden', textOverflow: 'ellipsis', whiteSpace: 'nowrap' }}>
+                              {b.username}
+                            </span>
+                            <span style={{
+                              fontSize: 11, fontWeight: 600, flexShrink: 0,
+                              color: b.balance > 0 ? '#4ade80' : '#f87171',
+                            }}>
+                              {b.balance > 0 ? '+' : ''}{fmt(b.balance, currency)}
+                            </span>
+                          </div>
+                        ))}
+                      </div>
+                    )}
+                  </div>
+                )}
+              </div>
+            )}
           </div>
 
           {pieSegments.length > 0 && (
@@ -641,27 +756,19 @@ export default function BudgetPanel({ tripId, tripMembers = [] }: BudgetPanelPro
 
               <PieChart segments={pieSegments} size={180} totalLabel={t('budget.total')} />
 
-              <div style={{ marginTop: 20, display: 'flex', flexDirection: 'column', gap: 8 }}>
+              <div style={{ marginTop: 20, display: 'flex', flexDirection: 'column', gap: 6 }}>
                 {pieSegments.map(seg => {
                   const pct = grandTotal > 0 ? ((seg.value / grandTotal) * 100).toFixed(1) : '0.0'
                   return (
                     <div key={seg.name} style={{ display: 'flex', alignItems: 'center', gap: 8 }}>
                       <div style={{ width: 10, height: 10, borderRadius: 3, background: seg.color, flexShrink: 0 }} />
                       <span style={{ flex: 1, fontSize: 12, color: 'var(--text-secondary)', fontWeight: 500 }}>{seg.name}</span>
-                      <span style={{ fontSize: 12, color: 'var(--text-muted)', fontWeight: 600, whiteSpace: 'nowrap' }}>{pct}%</span>
+                      <span style={{ fontSize: 11, color: 'var(--text-faint)', fontWeight: 600, whiteSpace: 'nowrap' }}>{fmt(seg.value, currency)}</span>
+                      <span style={{ fontSize: 11, color: 'var(--text-muted)', fontWeight: 600, whiteSpace: 'nowrap', minWidth: 38, textAlign: 'right' }}>{pct}%</span>
                     </div>
                   )
                 })}
               </div>
-
-              <div style={{ marginTop: 12, borderTop: '1px solid var(--border-secondary)', paddingTop: 12, display: 'flex', flexDirection: 'column', gap: 6 }}>
-                {pieSegments.map(seg => (
-                  <div key={seg.name} style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center' }}>
-                    <span style={{ fontSize: 11, color: 'var(--text-faint)' }}>{seg.name}</span>
-                    <span style={{ fontSize: 12, color: 'var(--text-secondary)', fontWeight: 600 }}>{fmt(seg.value, currency)}</span>
-                  </div>
-                ))}
-              </div>
             </div>
           )}
 

client/src/i18n/translations/de.ts

@@ -800,6 +800,9 @@ const de: Record<string, string | { name: string; category: string }[]> = {
   'budget.paid': 'Bezahlt',
   'budget.open': 'Offen',
   'budget.noMembers': 'Keine Teilnehmer zugewiesen',
+  'budget.settlement': 'Ausgleich',
+  'budget.settlementInfo': 'Klicke auf ein Mitglied-Bild bei einem Eintrag, um es grün zu markieren — das bedeutet, diese Person hat bezahlt. Der Ausgleich zeigt dann, wer wem wie viel schuldet.',
+  'budget.netBalances': 'Netto-Salden',
 
   // Files
   'files.title': 'Dateien',

client/src/i18n/translations/en.ts

@@ -800,6 +800,9 @@ const en: Record<string, string | { name: string; category: string }[]> = {
   'budget.paid': 'Paid',
   'budget.open': 'Open',
   'budget.noMembers': 'No members assigned',
+  'budget.settlement': 'Settlement',
+  'budget.settlementInfo': 'Click a member avatar on a budget item to mark them green — this means they paid. The settlement then shows who owes whom and how much.',
+  'budget.netBalances': 'Net Balances',
 
   // Files
   'files.title': 'Files',

server/src/db/migrations.ts

@@ -325,6 +325,10 @@ function runMigrations(db: Database.Database): void {
       // Add day_plan_position to reservations for persistent transport ordering in day timeline
       try { db.exec('ALTER TABLE reservations ADD COLUMN day_plan_position REAL DEFAULT NULL'); } catch {}
     },
+    () => {
+      // Add paid_by_user_id to budget_items for expense tracking / settlement
+      try { db.exec('ALTER TABLE budget_items ADD COLUMN paid_by_user_id INTEGER REFERENCES users(id)'); } catch {}
+    },
   ];
 
   if (currentVersion < migrations.length) {

server/src/routes/budget.ts

@@ -195,6 +195,77 @@ router.put('/:id/members/:userId/paid', authenticate, (req: Request, res: Respon
   broadcast(Number(tripId), 'budget:member-paid-updated', { itemId: Number(id), userId: Number(userId), paid: paid ? 1 : 0 }, req.headers['x-socket-id'] as string);
 });
 
+// Settlement calculation: who owes whom
+router.get('/settlement', authenticate, (req: Request, res: Response) => {
+  const authReq = req as AuthRequest;
+  const { tripId } = req.params;
+  if (!canAccessTrip(Number(tripId), authReq.user.id)) return res.status(404).json({ error: 'Trip not found' });
+
+  const items = db.prepare('SELECT * FROM budget_items WHERE trip_id = ?').all(tripId) as BudgetItem[];
+  const allMembers = db.prepare(`
+    SELECT bm.budget_item_id, bm.user_id, bm.paid, u.username, u.avatar
+    FROM budget_item_members bm
+    JOIN users u ON bm.user_id = u.id
+    WHERE bm.budget_item_id IN (SELECT id FROM budget_items WHERE trip_id = ?)
+  `).all(tripId) as (BudgetItemMember & { budget_item_id: number })[];
+
+  // Calculate net balance per user: positive = is owed money, negative = owes money
+  const balances: Record<number, { user_id: number; username: string; avatar_url: string | null; balance: number }> = {};
+
+  for (const item of items) {
+    const members = allMembers.filter(m => m.budget_item_id === item.id);
+    if (members.length === 0) continue;
+
+    const payers = members.filter(m => m.paid);
+    if (payers.length === 0) continue; // no one marked as paid
+
+    const sharePerMember = item.total_price / members.length;
+    const paidPerPayer = item.total_price / payers.length;
+
+    for (const m of members) {
+      if (!balances[m.user_id]) {
+        balances[m.user_id] = { user_id: m.user_id, username: m.username, avatar_url: avatarUrl(m), balance: 0 };
+      }
+      // Everyone owes their share
+      balances[m.user_id].balance -= sharePerMember;
+      // Payers get credited what they paid
+      if (m.paid) balances[m.user_id].balance += paidPerPayer;
+    }
+  }
+
+  // Calculate optimized payment flows (greedy algorithm)
+  const people = Object.values(balances).filter(b => Math.abs(b.balance) > 0.01);
+  const debtors = people.filter(p => p.balance < -0.01).map(p => ({ ...p, amount: -p.balance }));
+  const creditors = people.filter(p => p.balance > 0.01).map(p => ({ ...p, amount: p.balance }));
+
+  // Sort by amount descending for efficient matching
+  debtors.sort((a, b) => b.amount - a.amount);
+  creditors.sort((a, b) => b.amount - a.amount);
+
+  const flows: { from: { user_id: number; username: string; avatar_url: string | null }; to: { user_id: number; username: string; avatar_url: string | null }; amount: number }[] = [];
+
+  let di = 0, ci = 0;
+  while (di < debtors.length && ci < creditors.length) {
+    const transfer = Math.min(debtors[di].amount, creditors[ci].amount);
+    if (transfer > 0.01) {
+      flows.push({
+        from: { user_id: debtors[di].user_id, username: debtors[di].username, avatar_url: debtors[di].avatar_url },
+        to: { user_id: creditors[ci].user_id, username: creditors[ci].username, avatar_url: creditors[ci].avatar_url },
+        amount: Math.round(transfer * 100) / 100,
+      });
+    }
+    debtors[di].amount -= transfer;
+    creditors[ci].amount -= transfer;
+    if (debtors[di].amount < 0.01) di++;
+    if (creditors[ci].amount < 0.01) ci++;
+  }
+
+  res.json({
+    balances: Object.values(balances).map(b => ({ ...b, balance: Math.round(b.balance * 100) / 100 })),
+    flows,
+  });
+});
+
 router.delete('/:id', authenticate, (req: Request, res: Response) => {
   const authReq = req as AuthRequest;
   const { tripId, id } = req.params;