fix(realtime): доставлять обложку альбома после готовности вариантов

Карточка альбома не получала ссылку на обложку: она появляется в объекте
альбома только после генерации thumb/medium, а событие об этом шаге в
персональные потоки не публиковалось — счётчик обновлялся, обложка нет.

- albums.Service.NotifyCoverReady публикует album.updated получателям, когда
  варианты готовы у фото, являющегося текущей обложкой (иначе no-op, чтобы
  массовая загрузка не рассылала событие на каждый файл)
- photos: порт AlbumAccess расширен, worker вызывает публикацию после
  photo.updated
- project.md §11/§12: триггер добавлен в контракт персонального потока

backend/internal/albums/service.go

@@ -80,6 +80,23 @@ func (s *Service) NotifyAlbumChanged(ctx context.Context, albumID string) {
 	s.PublishAlbumTo(ctx, albumID, EventAlbumUpdated, s.albumAudience(ctx, albumID))
 }
 
+// NotifyCoverReady republishes the album to personal streams when the photo that
+// is currently its cover gains its variant URLs. The album object only carries a
+// cover link once thumb/medium exist, so without this event an album card keeps
+// showing a placeholder. Readiness of any other photo does not change the album
+// object and is ignored, which keeps bulk uploads from fanning out an event per
+// file.
+func (s *Service) NotifyCoverReady(ctx context.Context, albumID, photoID string) {
+	a, err := s.store.GetByID(ctx, albumID)
+	if err != nil {
+		return
+	}
+	if a.CoverPhotoID == nil || *a.CoverPhotoID != photoID {
+		return
+	}
+	s.NotifyAlbumChanged(ctx, albumID)
+}
+
 // albumAudience resolves the users an album is visible to. Resolution failures
 // are swallowed: a missed fan-out is recovered by the client's reconnect sync
 // and must not fail the mutation that triggered it.

backend/internal/albums/service_test.go

@@ -314,7 +314,57 @@ func TestNotifyAlbumChangedReachesRecipientsOnly(t *testing.T) {
 	assertNoEvent(t, albumSub)
 }
 
-// photos_count counts only photos outside the trash.
+// Cover readiness republishes the album only when the photo that became ready is
+// the current cover; any other photo leaves the album object unchanged.
+func TestNotifyCoverReadyPublishesOnlyForTheCover(t *testing.T) {
+	ctx := context.Background()
+	st := newStack(t)
+	owner := seedUser(t, st, "Owner")
+	member := seedUser(t, st, "Member")
+
+	album, err := st.svc.Create(ctx, owner, albums.CreateInput{Title: "Cover"})
+	if err != nil {
+		t.Fatalf("create: %v", err)
+	}
+	if err := st.repos.Members.Add(ctx, album.ID, member, "member", &owner); err != nil {
+		t.Fatalf("add member: %v", err)
+	}
+
+	const coverID = "photo-cover"
+	if _, err := st.db.Exec(
+		`INSERT INTO photos (id, album_id, uploader_id) VALUES ($1, $2, $3)`,
+		coverID, album.ID, owner,
+	); err != nil {
+		t.Fatalf("seed cover photo: %v", err)
+	}
+	if _, err := st.db.Exec(
+		`UPDATE albums SET cover_photo_id = $1 WHERE id = $2`, coverID, album.ID,
+	); err != nil {
+		t.Fatalf("set cover: %v", err)
+	}
+
+	ownerSub := st.hub.SubscribeUser(owner)
+	defer ownerSub.Close()
+	memberSub := st.hub.SubscribeUser(member)
+	defer memberSub.Close()
+
+	// A photo that is not the cover changes nothing.
+	st.svc.NotifyCoverReady(ctx, album.ID, "photo-other")
+	assertNoEvent(t, ownerSub)
+	assertNoEvent(t, memberSub)
+
+	// The cover becoming ready reaches the owner and every member.
+	st.svc.NotifyCoverReady(ctx, album.ID, coverID)
+	for name, sub := range map[string]*sse.Subscription{"owner": ownerSub, "member": memberSub} {
+		ev := nextEvent(t, sub)
+		if ev.Type != albums.EventAlbumUpdated {
+			t.Fatalf("%s event = %q, want %q", name, ev.Type, albums.EventAlbumUpdated)
+		}
+		if a, ok := ev.Data.(*models.Album); !ok || a.ID != album.ID {
+			t.Fatalf("%s payload = %#v", name, ev.Data)
+		}
+	}
+}
 func TestPhotosCountExcludesTrash(t *testing.T) {
 	ctx := context.Background()
 	st := newStack(t)