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-lock.json

@@ -9,6 +9,7 @@
       "version": "0.0.0",
       "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",
@@ -3400,6 +3401,15 @@
         "node": ">=6"
       }
     },
+    "node_modules/qrcode.react": {
+      "version": "4.2.0",
+      "resolved": "https://registry.npmjs.org/qrcode.react/-/qrcode.react-4.2.0.tgz",
+      "integrity": "sha512-QpgqWi8rD9DsS9EP3z7BT+5lY5SFhsqGjpgW5DY/i3mK4M9DTBNz3ErMi8BWYEfI3L0d8GIbGmcdFAS1uIRGjA==",
+      "license": "ISC",
+      "peerDependencies": {
+        "react": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0"
+      }
+    },
     "node_modules/queue-microtask": {
       "version": "1.2.3",
       "resolved": "https://registry.npmjs.org/queue-microtask/-/queue-microtask-1.2.3.tgz",