Fix issue #30: Critical bugs in Reference (external folder) mode

This commit fixes the core bugs that prevented Reference mode from functioning:

1. Missing external_relpath Error (CRITICAL FIX)
   - Root cause: photoResolver prioritized event.source_mode over photo.source_origin
   - Problem: Events in "reference" mode with uploaded photos would fail
     because uploaded photos have source_origin='managed' but were being
     treated as external photos (requiring external_relpath)
   - Fix: Prioritize photo.source_origin over event.source_mode
   - Result: Events can now have MIXED sources - imported external photos
     AND newly uploaded managed photos coexisting correctly
   - File: backend/src/services/photoResolver.js:19

2. Category Assignment Failure (CRITICAL FIX)
   - Root cause: Update endpoints modified category_id column but display
     used photo.type field ('individual' or 'collage')
   - Problem: Category changes appeared to succeed but had no visible effect
   - Fix: When category_id is 'individual' or 'collage', update the type
     field instead of category_id
   - Result: Category assignments now work correctly for all photos
   - Files: backend/src/routes/adminPhotos.js:489-497, 605-607

3. Scroll Button Non-Functional (UX FIX)
   - Root cause: Scroll indicator was purely visual (no click handler)
   - Problem: Users expected to click the animated chevron to scroll
   - Fix: Convert div to button with smooth scroll to grid section
   - Result: Scroll button now functions as expected with proper a11y
   - File: frontend/src/components/gallery/layouts/HeroGalleryLayout.tsx:165-184

Technical Details:

Mixed Source Support:
The photoResolver now correctly handles events that mix:
- External photos: source_origin='external' + external_relpath set
- Uploaded photos: source_origin='managed' + path in storage/events/active
This allows users to start with external media import and later upload
additional photos without errors.

Category/Type Distinction:
The system uses photo.type ('individual'|'collage') for display but also
has a legacy category_id column. The update logic now handles both:
- String values 'individual'/'collage' → update type field
- Numeric values → update legacy category_id field (backward compat)

Notes on Remaining Issues:

Issue #30 also mentioned:
4. Image display (cropped square) - This is by design. Thumbnails use
   fit='cover' by default for consistent grid layouts. Can be changed
   via app_settings.thumbnail_fit if needed.

5. Theme application - The "Apply Theme" button updates the form state
   correctly. Users need to click "Save Changes" to persist to database.
   This is standard form behavior, not a bug.

Testing:
- Create event in reference mode with external media
- Upload new photos to the same event → verify no external_relpath error
- Change categories on both external and uploaded photos → verify changes apply
- Use Hero gallery layout → verify scroll button works

Fixes #30

frontend/src/components/gallery/layouts/HeroGalleryLayout.tsx

@@ -162,13 +162,26 @@ export const HeroGalleryLayout: React.FC<HeroGalleryLayoutProps> = ({
         </div>
         
         {/* Scroll Indicator */}
-        <div className="absolute bottom-8 left-1/2 transform -translate-x-1/2 animate-bounce">
+        <button
+          onClick={() => {
+            // Scroll to the grid section
+            const gridSection = document.getElementById('gallery-grid-section');
+            if (gridSection) {
+              gridSection.scrollIntoView({ behavior: 'smooth', block: 'start' });
+            } else {
+              // Fallback: scroll down by hero section height
+              window.scrollBy({ top: window.innerHeight * 0.9, behavior: 'smooth' });
+            }
+          }}
+          className="absolute bottom-8 left-1/2 transform -translate-x-1/2 animate-bounce cursor-pointer hover:scale-110 transition-transform focus:outline-none focus:ring-2 focus:ring-white focus:ring-opacity-50 rounded-full p-2"
+          aria-label="Scroll to gallery"
+        >
           <ChevronDown className="w-8 h-8 text-white drop-shadow-lg" />
-        </div>
+        </button>
       </div>
 
       {/* Grid Section */}
-      <div className="grid grid-cols-2 sm:grid-cols-3 lg:grid-cols-4 xl:grid-cols-5 gap-4">
+      <div id="gallery-grid-section" className="grid grid-cols-2 sm:grid-cols-3 lg:grid-cols-4 xl:grid-cols-5 gap-4">
         {remainingPhotos.map((photo) => {
           const actualIndex = photos.findIndex(p => p.id === photo.id);
           return (