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/actions/routes.py

@@ -0,0 +1,79 @@
+"""Blog app action endpoints.
+
+Exposes write operations at ``/internal/actions/<action_name>`` for
+cross-app callers via the internal action client.
+"""
+from __future__ import annotations
+
+from quart import Blueprint, g, jsonify, request
+
+from shared.infrastructure.actions import ACTION_HEADER
+
+
+def register() -> Blueprint:
+    bp = Blueprint("actions", __name__, url_prefix="/internal/actions")
+
+    @bp.before_request
+    async def _require_action_header():
+        if not request.headers.get(ACTION_HEADER):
+            return jsonify({"error": "forbidden"}), 403
+
+    _handlers: dict[str, object] = {}
+
+    @bp.post("/<action_name>")
+    async def handle_action(action_name: str):
+        handler = _handlers.get(action_name)
+        if handler is None:
+            return jsonify({"error": "unknown action"}), 404
+        result = await handler()
+        return jsonify(result or {"ok": True})
+
+    # --- update-page-config ---
+    async def _update_page_config():
+        """Create or update a PageConfig (used by events payment admin)."""
+        from shared.models.page_config import PageConfig
+        from sqlalchemy import select
+
+        data = await request.get_json(force=True)
+        container_type = data.get("container_type", "page")
+        container_id = data.get("container_id")
+        if container_id is None:
+            return {"error": "container_id required"}, 400
+
+        pc = (await g.s.execute(
+            select(PageConfig).where(
+                PageConfig.container_type == container_type,
+                PageConfig.container_id == container_id,
+            )
+        )).scalar_one_or_none()
+
+        if pc is None:
+            pc = PageConfig(
+                container_type=container_type,
+                container_id=container_id,
+                features=data.get("features", {}),
+            )
+            g.s.add(pc)
+            await g.s.flush()
+
+        if "sumup_merchant_code" in data:
+            pc.sumup_merchant_code = data["sumup_merchant_code"] or None
+        if "sumup_checkout_prefix" in data:
+            pc.sumup_checkout_prefix = data["sumup_checkout_prefix"] or None
+        if "sumup_api_key" in data:
+            pc.sumup_api_key = data["sumup_api_key"] or None
+
+        await g.s.flush()
+
+        return {
+            "id": pc.id,
+            "container_type": pc.container_type,
+            "container_id": pc.container_id,
+            "sumup_merchant_code": pc.sumup_merchant_code,
+            "sumup_checkout_prefix": pc.sumup_checkout_prefix,
+            "sumup_configured": bool(pc.sumup_api_key),
+        }
+
+    _handlers["update-page-config"] = _update_page_config
+
+    return bp