Fix DTO compatibility: replace ORM relationship traversals with DTO fields

Templates were accessing entry.calendar.name/slug/post via ORM relationships,
but these entries are now CalendarEntryDTOs. Use flat fields instead
(calendar_name, calendar_slug, etc.).

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

services/calendar_impl.py

@@ -42,6 +42,8 @@ def _entry_to_dto(entry: CalendarEntry) -> CalendarEntryDTO:
         ticket_count=entry.ticket_count,
         calendar_name=cal.name if cal else None,
         calendar_slug=cal.slug if cal else None,
+        calendar_container_id=cal.container_id if cal else None,
+        calendar_container_type=cal.container_type if cal else None,
     )