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>

contracts/protocols.py

@@ -129,6 +129,11 @@ class CalendarService(Protocol):
         self, session: AsyncSession, content_type: str, content_id: int,
     ) -> set[int]: ...
 
+    async def upcoming_entries_for_container(
+        self, session: AsyncSession, container_type: str, container_id: int,
+        *, page: int = 1, per_page: int = 20,
+    ) -> tuple[list[CalendarEntryDTO], bool]: ...
+
     async def visible_entries_for_period(
         self, session: AsyncSession, calendar_id: int,
         period_start: datetime, period_end: datetime,