Implement event completion workflow (Task 14)

- Add completeEvent endpoint to backend API
- Create EventCompletionForm component with litter quantity input and photo upload
- Update EventDetailModal to show completion form for organizers
- Add isOrganizer check in EventsPage
- Update event service to support completion
- Display completion data (litter collected, photos) for completed events

server/src/controllers/eventController.ts

@@ -343,4 +343,66 @@ export const eventController = {
       });
     }
   },
+
+  // Complete event
+  async completeEvent(req: AuthRequest, res: Response) {
+    try {
+      const clerkId = req.userId;
+      if (!clerkId) {
+        return res.status(401).json({
+          code: 'UNAUTHORIZED',
+          message: 'User not authenticated',
+          timestamp: new Date().toISOString(),
+        });
+      }
+
+      const userId = await findOrCreateUser(clerkId);
+
+      const { id } = req.params;
+      const event = await eventService.getEventById(id);
+
+      if (!event) {
+        return res.status(404).json({
+          code: 'EVENT_NOT_FOUND',
+          message: 'Event not found',
+          timestamp: new Date().toISOString(),
+        });
+      }
+
+      // Check if user is the organizer
+      if (event.organizerId !== userId) {
+        return res.status(403).json({
+          code: 'FORBIDDEN',
+          message: 'Only the event organizer can complete this event',
+          timestamp: new Date().toISOString(),
+        });
+      }
+
+      const { litterCollected, photos } = req.body;
+
+      // Validation
+      if (litterCollected === undefined || litterCollected === null) {
+        return res.status(400).json({
+          code: 'VALIDATION_ERROR',
+          message: 'litterCollected is required',
+          timestamp: new Date().toISOString(),
+        });
+      }
+
+      const completedEvent = await eventService.completeEvent(id, {
+        litterCollected: parseFloat(litterCollected),
+        photos: photos || [],
+      });
+
+      res.json(completedEvent);
+    } catch (error: any) {
+      console.error('Error completing event:', error);
+      res.status(500).json({
+        code: 'COMPLETE_EVENT_ERROR',
+        message: 'Failed to complete event',
+        details: error.message,
+        timestamp: new Date().toISOString(),
+      });
+    }
+  },
 };