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 <[email protected]>

backend/src/routes/gallery.js

@@ -157,7 +157,7 @@ router.get('/:slug/photos', verifyGalleryAccess, async (req, res) => {
         id: photo.id,
         filename: photo.filename,
         url: `/photos/${photo.path}`,
-        thumbnail_url: photo.thumbnail_path ? `/thumbnails/${path.basename(photo.thumbnail_path)}` : null,
+        thumbnail_url: photo.thumbnail_path ? `/${photo.thumbnail_path}` : null,
         type: photo.type,
         category_id: photo.category_id,
         category_name: photo.category_name,