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/services/api.js

@@ -199,6 +199,25 @@ export const eventsAPI = {
     const data = await fetchAPI(`/events/${slug}/messages?${params}`);
     return data;
   },
+
+  async getDetails(slug) {
+    const data = await fetchAPI(`/events/${slug}/details`);
+    return data;
+  },
+
+  async checkin(token) {
+    const data = await fetchAPI(`/events/checkin/${token}`, {
+      method: 'POST',
+    });
+    return data;
+  },
+
+  async leave(slug) {
+    const data = await fetchAPI(`/events/${slug}/leave`, {
+      method: 'DELETE',
+    });
+    return data;
+  },
 };
 
 export { ApiError };