fix(photos): faster large-album loading and album cover lifecycle

Large albums opened slowly and flickered; the default album cover was
also lost in the media-objects refactor.

Large-album loading:
- quantize signed object URLs within a window and send Cache-Control so
  previews are reused instead of re-fetched
- publish photo.updated (not photo.created) when variants are ready, so
  clients update in place without reordering the grid
- worker concurrency + newest-first variant priority; raise default batch

Album cover:
- first uploaded photo becomes the cover when none is set
- clear the cover when the cover photo is trashed or moved out
- "set as album cover" in the photo context menu and actions panel

OpenSpec changes: fix-large-album-load, fix-album-cover

backend/internal/photos/repository.go

@@ -28,6 +28,7 @@ type Repository interface {
 	DetachTag(ctx context.Context, photoID, tagID string) error
 	TagOwnedBy(ctx context.Context, tagID, userID string) (bool, error)
 	SetCoverIfEmpty(ctx context.Context, albumID, photoID string)
+	ClearCoverIfMatches(ctx context.Context, albumID, photoID string)
 	UpdateFilename(ctx context.Context, id, filename string) error
 	BackfillEXIF(ctx context.Context, fetch func(context.Context, string) ([]byte, error)) (int, error)