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/services/eventService.ts

@@ -324,4 +324,40 @@ export const eventService = {
 
     return !!registration;
   },
+
+  async completeEvent(eventId: string, data: { litterCollected: number; photos?: string[] }) {
+    const event = await prisma.event.update({
+      where: { id: eventId },
+      data: {
+        status: 'completed',
+        litterCollected: data.litterCollected,
+        photos: data.photos || [],
+      },
+      include: {
+        _count: {
+          select: {
+            registrations: true,
+          },
+        },
+      },
+    });
+
+    return {
+      id: event.id,
+      organizerId: event.organizerId,
+      title: event.title,
+      description: event.description,
+      latitude: parseFloat(event.latitude.toString()),
+      longitude: parseFloat(event.longitude.toString()),
+      locationName: event.locationName,
+      scheduledDate: event.scheduledDate,
+      duration: event.duration,
+      status: event.status || 'upcoming',
+      participantCount: event._count.registrations,
+      litterCollected: event.litterCollected ? parseFloat(event.litterCollected.toString()) : undefined,
+      photos: event.photos ? (Array.isArray(event.photos) ? event.photos : []) : undefined,
+      createdAt: event.createdAt,
+      updatedAt: event.updatedAt,
+    };
+  },
 };