feat: settings panel + thumbnail pipeline fixes

- photos.py: stop crashing in FileResponse when a thumb hasn't been
  generated; return a clean 404 with Retry-After so the frontend can
  back off.
- thumbs.py: fix process_video_thumbnail (overwrite_output, robust
  duration probe across stream/format, eager frame load + temp cleanup)
  so videos stop ending up as the gray placeholder.
- library.py: new /maintenance/* endpoints — thumbnail-stats,
  regenerate-thumbnails (with media_type / only_failed filters), and a
  manual data-integrity cleanup trigger.
- Frontend Settings panel (gear in TopBar) surfacing those endpoints
  plus a re-scan button and live thumbnail status counts.
- PhotoThumbnail: stretch the auto-retry schedule for slow RAW jobs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/routers/photos.py

@@ -322,12 +322,14 @@ async def get_thumbnail(
     thumb_path = f"{thumb_dir}/{size}.webp"
     
     if not os.path.exists(thumb_path):
-        # Generate thumbnail on demand
+        # Queue background generation (handles RAW/HEIC/video properly)
         from app.tasks.thumbs import generate_thumbnails
         generate_thumbnails.delay(photo_id)
-        
-        # For now, return a placeholder or the original with reduced quality
-        if os.path.exists(photo.filepath):
+
+        # Best-effort inline fallback for standard images so the first
+        # request doesn't have to wait for the worker. RAW/HEIC/video
+        # and missing source files fall through to a clean 404 below.
+        if photo.filepath and os.path.exists(photo.filepath):
             from PIL import Image
             try:
                 os.makedirs(thumb_dir, exist_ok=True)
@@ -359,10 +361,23 @@ async def get_thumbnail(
                 # Save as WebP
                 img.save(thumb_path, 'WEBP', quality=85, optimize=True)
                 
+            except HTTPException:
+                raise
             except Exception as e:
-                logger.error(f"Error generating thumbnail: {e}")
-                raise HTTPException(status_code=404, detail="Could not generate thumbnail")
-    
+                logger.warning(
+                    f"Inline thumbnail fallback failed for {photo_id} ({size}); "
+                    f"waiting on worker: {e}"
+                )
+
+    # If the inline fallback didn't (or couldn't) produce the file,
+    # tell the client to retry instead of crashing in FileResponse/nginx.
+    if not os.path.exists(thumb_path):
+        raise HTTPException(
+            status_code=404,
+            detail="Thumbnail not ready",
+            headers={"Retry-After": "2"},
+        )
+
     # Check if we're behind Nginx
     if os.environ.get('USE_X_ACCEL_REDIRECT'):
         # Use Nginx X-Accel-Redirect for better performance