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>

contracts/dtos.py

@@ -60,6 +60,8 @@ class CalendarEntryDTO:
     ticket_count: int | None = None
     calendar_name: str | None = None
     calendar_slug: str | None = None
+    calendar_container_id: int | None = None
+    calendar_container_type: str | None = None
 
 
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