fix: add missing images/ prefix in album cover URL (#346)

the album cover upload was constructing URLs without the images/ prefix,
causing 404s when trying to display uploaded album covers.

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Co-authored-by: Claude <[e-mail]>

backend/src/backend/api/albums.py

···
         # save returns the file_id (hash)
         image_id = await storage.save(image_obj, image.filename)
 
-        # construct R2 URL directly
-        # storage.save uses just {file_id}{ext} for images (no subdirectory)
-        image_url = f"{storage.public_image_bucket_url}/{image_id}{ext}"
+        # construct R2 URL directly (images are stored under images/ prefix)
+        image_url = f"{storage.public_image_bucket_url}/images/{image_id}{ext}"
 
         # delete old image if exists (prevent R2 object leaks)
         if album.image_id: