fix: delete stale cached albums that no longer exist in VK

After refresh_vk_catalog syncs the album list from VK, remove any
VkCachedAlbum rows whose album_id was not returned by the API.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/tasks/vk_catalog.py

@@ -62,6 +62,16 @@ def _sync_user(db, user_id: int, token: str, group_id: str) -> None:
                 count=a.get("count"),
                 fetched_at=now,
             ))
+    # Delete cached albums that no longer exist in VK
+    (
+        db.query(VkCachedAlbum)
+        .filter(
+            VkCachedAlbum.user_id == user_id,
+            VkCachedAlbum.vk_group_id == group_id,
+            VkCachedAlbum.album_id.notin_(album_ids),
+        )
+        .delete(synchronize_session=False)
+    )
     db.flush()
 
     # ── products (extended=1 gives albums_ids per product) ───────────────────