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>

services/stubs.py

@@ -156,6 +156,13 @@ class StubMarketService:
     ) -> list[MarketPlaceDTO]:
         return []
 
+    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]:
+        return [], False
+
     async def product_by_id(self, session: AsyncSession, product_id: int) -> ProductDTO | None:
         return None