Add MarketService write methods and clean up stubs

Add create_marketplace() and soft_delete_marketplace() to MarketService
protocol, SQL implementation, and stubs — centralises market CRUD that
was previously duplicated in blog and events app-level service files.

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

contracts/protocols.py

@@ -78,6 +78,16 @@ class MarketService(Protocol):
 
     async def product_by_id(self, session: AsyncSession, product_id: int) -> ProductDTO | None: ...
 
+    async def create_marketplace(
+        self, session: AsyncSession, container_type: str, container_id: int,
+        name: str, slug: str,
+    ) -> MarketPlaceDTO: ...
+
+    async def soft_delete_marketplace(
+        self, session: AsyncSession, container_type: str, container_id: int,
+        slug: str,
+    ) -> bool: ...
+
 
 @runtime_checkable
 class CartService(Protocol):