feat: add QR code event check-in system

Backend:
- Add event_checkin_tokens table with unique tokens per event
- Implement GET /api/events/:slug/details endpoint (on-demand token generation)
- Implement POST /api/events/checkin/:token endpoint (date validation only in production)
- Implement DELETE /api/events/:slug/leave endpoint
- Add comprehensive test suite for check-in endpoints

Frontend:
- Add EventDetailsPage with QR code display, participant list, and stats
- Add EventCheckinPage with success/error screens
- Add "Leave Event" button with confirmation modal to EventChatPage
- Install qrcode.react library for QR code generation
- Update routing and API client with new endpoints

Features:
- QR codes valid from (startDate-1d) to (endDate+1d)
- Development mode bypasses date validation for testing
- Automatic participant count tracking
- Duplicate check-in prevention
- Token reuse for same event (generated once, cached)

frontend/package.json

@@ -11,6 +11,7 @@
   },
   "dependencies": {
     "lucide-react": "^0.553.0",
+    "qrcode.react": "^4.2.0",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
     "react-router-dom": "^7.9.5",