refactor(mcp): replace direct DB access with service layer calls

Replace all db.prepare() calls in mcp/index.ts, mcp/resources.ts, and
mcp/tools.ts with calls to the service layer. Add missing service functions:
- authService: isDemoUser, verifyMcpToken, verifyJwtToken
- adminService: isAddonEnabled
- atlasService: listVisitedCountries
- tripService: getTripSummary, listTrips with null archived param

Also fix getAssignmentWithPlace and formatAssignmentWithPlace to expose
place_id, assignment_time, and assignment_end_time at the top level, and
fix updateDay to correctly handle null title for clearing.

Add comprehensive unit and integration test suite for the MCP layer (821 tests all passing).

server/src/services/adminService.ts

@@ -463,6 +463,11 @@ export function deleteTemplateItem(itemId: string) {
 
 // ── Addons ─────────────────────────────────────────────────────────────────
 
+export function isAddonEnabled(addonId: string): boolean {
+  const addon = db.prepare('SELECT enabled FROM addons WHERE id = ?').get(addonId) as { enabled: number } | undefined;
+  return !!addon?.enabled;
+}
+
 export function listAddons() {
   const addons = db.prepare('SELECT * FROM addons ORDER BY sort_order, id').all() as Addon[];
   return addons.map(a => ({ ...a, enabled: !!a.enabled, config: JSON.parse(a.config || '{}') }));