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/src/App.jsx

@@ -7,6 +7,8 @@ import ForgotPasswordPage from './pages/ForgotPasswordPage';
 import ResetPasswordPage from './pages/ResetPasswordPage';
 import EventsPage from './pages/EventsPage';
 import EventChatPage from './pages/EventChatPage';
+import EventDetailsPage from './pages/EventDetailsPage';
+import EventCheckinPage from './pages/EventCheckinPage';
 import MatchChatPage from './pages/MatchChatPage';
 import RatePartnerPage from './pages/RatePartnerPage';
 import HistoryPage from './pages/HistoryPage';
@@ -100,6 +102,22 @@ function App() {
               </ProtectedRoute>
             }
           />
+          <Route
+            path="/events/:slug/details"
+            element={
+              <ProtectedRoute>
+                <EventDetailsPage />
+              </ProtectedRoute>
+            }
+          />
+          <Route
+            path="/events/checkin/:token"
+            element={
+              <ProtectedRoute>
+                <EventCheckinPage />
+              </ProtectedRoute>
+            }
+          />
           <Route
             path="/matches/:matchId/chat"
             element={