feat(photos): add square medium variant for sharp grid thumbnails

backend/internal/albums/service.go

@@ -61,13 +61,27 @@ func (s *Service) List(ctx context.Context, userID string) ([]models.Album, erro
 	return out, nil
 }
 
-// attachCover fills CoverThumbURL from the signed thumbnail URL, when present.
+// attachCover fills CoverThumbURL and CoverMediumURL from signed URLs. The
+// medium URL falls back to the thumbnail while the medium variant is not ready,
+// so a cover URL suitable for large display is always present when a cover is
+// set.
 func (s *Service) attachCover(ctx context.Context, a *models.Album) {
-	if s.storage == nil || a == nil || a.CoverThumbKey == "" {
+	if s.storage == nil || a == nil {
 		return
 	}
-	if url, err := s.storage.URL(ctx, a.CoverThumbKey); err == nil {
-		a.CoverThumbURL = url
+	if a.CoverThumbKey != "" {
+		if url, err := s.storage.URL(ctx, a.CoverThumbKey); err == nil {
+			a.CoverThumbURL = url
+		}
+	}
+	key := a.CoverMediumKey
+	if key == "" {
+		key = a.CoverThumbKey
+	}
+	if key != "" {
+		if url, err := s.storage.URL(ctx, key); err == nil {
+			a.CoverMediumURL = url
+		}
 	}
 }