Add upcoming_entries_for_container to CalendarService

New paginated query for upcoming confirmed entries across all calendars
belonging to a container (page). Used by the events page summary view.

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

services/stubs.py

@@ -140,6 +140,9 @@ class StubCalendarService:
     ) -> int:
         return 0
 
+    async def upcoming_entries_for_container(self, session, container_type, container_id, *, page=1, per_page=20):
+        return [], False
+
     async def entry_ids_for_content(self, session, content_type, content_id):
         return set()