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/storage/storage.go

@@ -108,7 +108,7 @@ func Open(ctx context.Context, cfg *config.Config, jwtSecret string) (Backend, e
 	if secret == "" {
 		return nil, errors.New("object URL secret is not available")
 	}
-	signer, err := NewSigner(cfg.Server.BaseURL, secret, cfg.Storage.URLTTL)
+	signer, err := NewSigner(cfg.Server.BaseURL, secret, cfg.Storage.URLTTL, cfg.Storage.URLSignWindow)
 	if err != nil {
 		return nil, err
 	}