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/helpers_test.go

@@ -106,7 +106,7 @@ func newTestService(t *testing.T, pool *pgxpool.Pool) (*Service, storage.Backend
 	if endpoint == "" {
 		t.Skip("S3_ENDPOINT not set")
 	}
-	signer, err := storage.NewSigner("http://app.test", "test-secret", time.Hour)
+	signer, err := storage.NewSigner("http://app.test", "test-secret", time.Hour, 10*time.Minute)
 	if err != nil {
 		t.Skipf("signer init: %v", err)
 	}
@@ -128,7 +128,7 @@ func newTestService(t *testing.T, pool *pgxpool.Pool) (*Service, storage.Backend
 // storage backend so the full flow can run without external infrastructure.
 func newTestServiceFS(t *testing.T, pool *pgxpool.Pool) (*Service, storage.Backend) {
 	t.Helper()
-	signer, err := storage.NewSigner("http://app.test", "test-secret", time.Hour)
+	signer, err := storage.NewSigner("http://app.test", "test-secret", time.Hour, 10*time.Minute)
 	if err != nil {
 		t.Fatalf("storage init: %v", err)
 	}