Fix blog: add page_configs migration, fix stale cart reference in ghost_sync

- Add 0003_add_page_configs.py migration to create table in db_blog
- Fix ghost_sync.py: fetch_data("cart", "page-config-ensure") → "blog"

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

blog/bp/blog/ghost/ghost_sync.py

@@ -248,10 +248,10 @@ async def _upsert_post(sess: AsyncSession, gp: Dict[str, Any], author_map: Dict[
     finally:
         sess.autoflush = old_autoflush
 
-    # Auto-create PageConfig for pages (lives in db_cart, accessed via internal API)
+    # Auto-create PageConfig for pages (blog now owns this table)
     if obj.is_page:
         await fetch_data(
-            "cart", "page-config-ensure",
+            "blog", "page-config-ensure",
             params={"container_type": "page", "container_id": obj.id},
             required=False,
         )