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>

shared/models/order.py

@@ -69,14 +69,8 @@ class Order(Base):
         cascade="all, delete-orphan",
         lazy="selectin",
     )
-    # Cross-domain relationship — explicit join, viewonly (no FK constraint)
-    page_config: Mapped[Optional["PageConfig"]] = relationship(
-        "PageConfig",
-        primaryjoin="Order.page_config_id == PageConfig.id",
-        foreign_keys="[Order.page_config_id]",
-        viewonly=True,
-        lazy="selectin",
-    )
+    # page_config_id references PageConfig in db_blog (cross-domain).
+    # Fetch via HTTP: fetch_data("blog", "page-config-by-id", params={"id": ...})
 
 
 class OrderItem(Base):