Decoupling audit cleanup: fix protocol gaps, remove dead APIs

- Add search_posts, entry_ids_for_content, visible_entries_for_period
  to protocols and stubs
- Delete internal_api.py and factory cleanup hook (zero callers)
- Convert utils.py to utils/ package with calendar_helpers module
- Remove deleted_at check from calendar_view template (service filters)

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

services/stubs.py

@@ -31,6 +31,9 @@ class StubBlogService:
     async def get_posts_by_ids(self, session: AsyncSession, ids: list[int]) -> list[PostDTO]:
         return []
 
+    async def search_posts(self, session, query, page=1, per_page=10):
+        return [], 0
+
 
 class StubCalendarService:
     async def calendars_for_container(
@@ -129,6 +132,12 @@ class StubCalendarService:
     ) -> None:
         pass
 
+    async def entry_ids_for_content(self, session, content_type, content_id):
+        return set()
+
+    async def visible_entries_for_period(self, session, calendar_id, period_start, period_end, *, user_id, is_admin, session_id):
+        return []
+
 
 class StubMarketService:
     async def marketplaces_for_container(