Decouple PageConfig cross-domain queries + merge cart into db_market

PageConfig (db_blog) decoupling:
- Blog: add page-config, page-config-by-id, page-configs-batch data endpoints
- Blog: add update-page-config action endpoint for events payment admin
- Cart: hydrate_page, resolve_page_config, get_cart_grouped_by_page all
  fetch PageConfig from blog via HTTP instead of direct DB query
- Cart: check_sumup_status auto-fetches page_config from blog when needed
- Events: payment routes read/write PageConfig via blog HTTP endpoints
- Order model: remove cross-domain page_config ORM relationship (keep column)

Cart + Market DB merge:
- Cart tables (cart_items, orders, order_items) moved into db_market
- Cart app DATABASE_URL now points to db_market (same bounded context)
- CartItem.product / CartItem.market_place relationships work again
  (same database, no cross-domain join issues)
- Updated split-databases.sh, init-databases.sql, docker-compose.yml

Ghost sync fix:
- Wrap PostAuthor/PostTag delete+re-add in no_autoflush block
- Use synchronize_session="fetch" to keep identity map consistent
- Prevents query-invoked autoflush IntegrityError on composite PK

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

blog/bp/blog/ghost/ghost_sync.py

@@ -185,25 +185,34 @@ async def _upsert_post(sess: AsyncSession, gp: Dict[str, Any], author_map: Dict[
                 obj.user_id = user_id
                 await sess.flush()
 
-    # rebuild post_authors (dedup to avoid composite PK conflicts from Ghost dupes)
-    await sess.execute(delete(PostAuthor).where(PostAuthor.post_id == obj.id))
-    await sess.flush()
-    seen_authors: set[int] = set()
-    for idx, a in enumerate(gp.get("authors") or []):
-        aa = author_map[a["id"]]
-        if aa.id not in seen_authors:
-            seen_authors.add(aa.id)
-            sess.add(PostAuthor(post_id=obj.id, author_id=aa.id, sort_order=idx))
+    # Rebuild post_authors + post_tags inside no_autoflush to prevent
+    # premature INSERT from query-invoked autoflush.
+    async with sess.no_autoflush:
+        # Delete + re-add post_authors (dedup for Ghost duplicate authors)
+        await sess.execute(
+            delete(PostAuthor).where(PostAuthor.post_id == obj.id),
+            execution_options={"synchronize_session": "fetch"},
+        )
+        seen_authors: set[int] = set()
+        for idx, a in enumerate(gp.get("authors") or []):
+            aa = author_map[a["id"]]
+            if aa.id not in seen_authors:
+                seen_authors.add(aa.id)
+                sess.add(PostAuthor(post_id=obj.id, author_id=aa.id, sort_order=idx))
+
+        # Delete + re-add post_tags (dedup similarly)
+        await sess.execute(
+            delete(PostTag).where(PostTag.post_id == obj.id),
+            execution_options={"synchronize_session": "fetch"},
+        )
+        seen_tags: set[int] = set()
+        for idx, t in enumerate(gp.get("tags") or []):
+            tt = tag_map[t["id"]]
+            if tt.id not in seen_tags:
+                seen_tags.add(tt.id)
+                sess.add(PostTag(post_id=obj.id, tag_id=tt.id, sort_order=idx))
 
-    # rebuild post_tags (dedup similarly)
-    await sess.execute(delete(PostTag).where(PostTag.post_id == obj.id))
     await sess.flush()
-    seen_tags: set[int] = set()
-    for idx, t in enumerate(gp.get("tags") or []):
-        tt = tag_map[t["id"]]
-        if tt.id not in seen_tags:
-            seen_tags.add(tt.id)
-            sess.add(PostTag(post_id=obj.id, tag_id=tt.id, sort_order=idx))
 
     # Auto-create PageConfig for pages
     if obj.is_page: