feat(photos): separate object storage from metadata layer

Move physical file facts out of the photos table into an objects ledger:

- objects / photo_objects / variant_jobs tables (migration 000008)
- drop storage_key/thumb_key/preview_key/size/width/height/mime_type from
  photos (migration 000009); they are derived via joins to the original object
- album-free storage keys: objects/{id}/{role}.{ext}
- explicit GC via pending/ready/deleting states; objects deleted only when no
  photo references them
- background worker drives variant generation (durable variant_jobs) and GC
- copy/move are metadata-only; storage objects are shared between photos
- remove unused storage.DeletePrefix

Non-destructive to the external JSON API: size/dimensions/mime and URLs are
still returned, just sourced from the object layer.

OpenSpec change: media-object-layer

backend/internal/storage/storage.go

@@ -104,24 +104,6 @@ func (c *Client) Delete(ctx context.Context, keys ...string) error {
 	return nil
 }
 
-func (c *Client) DeletePrefix(ctx context.Context, prefix string) error {
-	var keys []string
-	paginator := s3.NewListObjectsV2Paginator(c.s3, &s3.ListObjectsV2Input{
-		Bucket: aws.String(c.bucket),
-		Prefix: aws.String(prefix),
-	})
-	for paginator.HasMorePages() {
-		page, err := paginator.NextPage(ctx)
-		if err != nil {
-			return fmt.Errorf("list objects: %w", err)
-		}
-		for _, o := range page.Contents {
-			keys = append(keys, aws.ToString(o.Key))
-		}
-	}
-	return c.Delete(ctx, keys...)
-}
-
 func (c *Client) Get(ctx context.Context, key string) ([]byte, error) {
 	out, err := c.s3.GetObject(ctx, &s3.GetObjectInput{
 		Bucket: aws.String(c.bucket),