fix: resolve image and thumbnail loading issues

- Fix static file serving paths to use correct storage directory
- Remove /api prefix from admin photo URLs to prevent double /api/api/ issue
- Fix thumbnail URL generation in gallery to use correct path format
- Update storage path resolution to support both relative and absolute paths

The issues were:
1. Admin images had URLs like /api/api/admin/events/2/thumbnail/90
2. Gallery thumbnails were looking for /thumbnails/thumb_*.jpg but paths were wrong
3. Static serving middleware was using incorrect storage paths

All images and thumbnails should now load correctly in both admin and gallery views.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/src/routes/adminPhotos.js

@@ -552,8 +552,8 @@ router.get('/:eventId/photos', adminAuth, async (req, res) => {
       photos: photos.map(photo => ({
         id: photo.id,
         filename: photo.filename,
-        url: `/api/admin/events/${eventId}/photo/${photo.id}`,
-        thumbnail_url: photo.thumbnail_path ? `/api/admin/events/${eventId}/thumbnail/${photo.id}` : null,
+        url: `/admin/events/${eventId}/photo/${photo.id}`,
+        thumbnail_url: photo.thumbnail_path ? `/admin/events/${eventId}/thumbnail/${photo.id}` : null,
         type: photo.type,
         category_id: photo.category_id,
         category_name: photo.category_name,