fix: thumbnail content-type, animated GIF preservation, and janitor cleanup

- Add extension-based mimetype fallback in uploads route so thumbnail
  files serve correct Content-Type (image/webp) instead of falling back
  to application/octet-stream when DB lookup misses
- Skip animated images (metadata.pages > 1) during thumbnail generation
  to preserve GIF/WebP animations instead of flattening to static frame
- Remove redundant explicit thumbnail deletion in storageJanitor since
  deleteUploadFile() already auto-deletes the thumbnail variant

packages/server/src/utils/thumbnail.ts

@@ -49,6 +49,11 @@ export async function generateThumbnail(
       return null;
     }
 
+    // Skip animated images — Sharp would flatten to a single static frame
+    if (metadata.pages && metadata.pages > 1) {
+      return null;
+    }
+
     const originalFilename = path.basename(originalPath);
     const thumbName = thumbFilename(originalFilename);
     const thumbPath = path.join(uploadDir, thumbName);