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/data/routes.py

@@ -7,9 +7,25 @@ from __future__ import annotations
 
 from quart import Blueprint, g, jsonify, request
 
+from sqlalchemy import select
+
 from shared.infrastructure.data_client import DATA_HEADER
 from shared.contracts.dtos import dto_to_dict
 from shared.services.registry import services
+from shared.models.page_config import PageConfig
+
+
+def _page_config_dict(pc: PageConfig) -> dict:
+    """Serialize PageConfig to a JSON-safe dict."""
+    return {
+        "id": pc.id,
+        "container_type": pc.container_type,
+        "container_id": pc.container_id,
+        "features": pc.features or {},
+        "sumup_merchant_code": pc.sumup_merchant_code,
+        "sumup_api_key": pc.sumup_api_key,
+        "sumup_checkout_prefix": pc.sumup_checkout_prefix,
+    }
 
 
 def register() -> Blueprint:
@@ -71,4 +87,56 @@ def register() -> Blueprint:
 
     _handlers["search-posts"] = _search_posts
 
+    # --- page-config ---
+    async def _page_config():
+        """Return a single PageConfig by container_type + container_id."""
+        ct = request.args.get("container_type", "page")
+        cid = request.args.get("container_id", type=int)
+        if cid is None:
+            return None
+        pc = (await g.s.execute(
+            select(PageConfig).where(
+                PageConfig.container_type == ct,
+                PageConfig.container_id == cid,
+            )
+        )).scalar_one_or_none()
+        if not pc:
+            return None
+        return _page_config_dict(pc)
+
+    _handlers["page-config"] = _page_config
+
+    # --- page-config-by-id ---
+    async def _page_config_by_id():
+        """Return a single PageConfig by its primary key."""
+        pc_id = request.args.get("id", type=int)
+        if pc_id is None:
+            return None
+        pc = await g.s.get(PageConfig, pc_id)
+        if not pc:
+            return None
+        return _page_config_dict(pc)
+
+    _handlers["page-config-by-id"] = _page_config_by_id
+
+    # --- page-configs-batch ---
+    async def _page_configs_batch():
+        """Return PageConfigs for multiple container_ids (comma-separated)."""
+        ct = request.args.get("container_type", "page")
+        ids_raw = request.args.get("ids", "")
+        if not ids_raw:
+            return []
+        ids = [int(x.strip()) for x in ids_raw.split(",") if x.strip()]
+        if not ids:
+            return []
+        result = await g.s.execute(
+            select(PageConfig).where(
+                PageConfig.container_type == ct,
+                PageConfig.container_id.in_(ids),
+            )
+        )
+        return [_page_config_dict(pc) for pc in result.scalars().all()]
+
+    _handlers["page-configs-batch"] = _page_configs_batch
+
     return bp