feat: photo rename and viewer delete with confirmation

Backend:
- add PATCH /api/photos/:id to rename original_filename (uploader/owner/admin)
- emit photo.updated SSE event

Frontend:
- inline filename editor in the right panel
- viewer: back button (arrow_back) on the left, delete on the right
- delete confirmed via a modal dialog; filmstrip updates after deletion

backend/internal/photos/store.go

@@ -127,3 +127,14 @@ func (s *Store) SetCoverIfEmpty(ctx context.Context, albumID, photoID string) {
 		`UPDATE albums SET cover_photo_id = $2 WHERE id = $1 AND cover_photo_id IS NULL`,
 		albumID, photoID)
 }
+
+func (s *Store) UpdateFilename(ctx context.Context, id, filename string) error {
+	tag, err := s.pool.Exec(ctx, `UPDATE photos SET original_filename = $2 WHERE id = $1`, id, filename)
+	if err != nil {
+		return err
+	}
+	if tag.RowsAffected() == 0 {
+		return ErrNotFound
+	}
+	return nil
+}