Add list_marketplaces to MarketService protocol, impl, and stub

Paginated query for market listings — supports optional container filtering
and returns (dtos, has_more) for infinite scroll.

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

contracts/protocols.py

@@ -155,6 +155,12 @@ class MarketService(Protocol):
         name: str, slug: str,
     ) -> MarketPlaceDTO: ...
 
+    async def list_marketplaces(
+        self, session: AsyncSession,
+        container_type: str | None = None, container_id: int | None = None,
+        *, page: int = 1, per_page: int = 20,
+    ) -> tuple[list[MarketPlaceDTO], bool]: ...
+
     async def soft_delete_marketplace(
         self, session: AsyncSession, container_type: str, container_id: int,
         slug: str,