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/copy_test.go

@@ -22,7 +22,7 @@ func TestCopySharesStorageObjects(t *testing.T) {
 	}
 	t.Cleanup(func() { cleanupPhoto(t, ctx, pool, cl, p.ID) })
 
-	w := NewWorker(svc, time.Second, 10, time.Minute)
+	w := NewWorker(svc, time.Second, 10, time.Minute, 2)
 	w.processVariantJobs(ctx)
 
 	srcIDs, err := svc.store.PhotoObjectIDs(ctx, p.ID)