Stabilize core: better-sqlite3, versioned migrations, graceful shutdown

- Replace experimental node:sqlite with better-sqlite3 (stable API)
- Replace try/catch migration pattern with schema_version tracking
- Add SIGTERM/SIGINT handler for clean shutdown (DB flush, scheduler stop)
- Fix BudgetPanel crash: remove undefined setShowAddCategory call
- Update Dockerfile: remove --experimental-sqlite, add native build tools

client/src/components/Budget/BudgetPanel.jsx

@@ -190,7 +190,7 @@ export default function BudgetPanel({ tripId }) {
   const handleAddCategory = () => {
     if (!newCategoryName.trim()) return
     addBudgetItem(tripId, { name: t('budget.defaultEntry'), category: newCategoryName.trim(), total_price: 0 })
-    setNewCategoryName(''); setShowAddCategory(false)
+    setNewCategoryName('')
   }
 
   const th = { padding: '6px 8px', textAlign: 'center', fontSize: 11, fontWeight: 600, color: 'var(--text-muted)', textTransform: 'uppercase', letterSpacing: '0.05em', borderBottom: '2px solid var(--border-primary)', whiteSpace: 'nowrap', background: 'var(--bg-secondary)' }