fix(tz): interpret calendar and briefing dates in user's local timezone

Two related bugs where the server defaulted naive datetimes to UTC instead
of the configured user timezone, causing all-day events to land on the
previous day and briefings to "disappear" at UTC midnight.

- New services/tz.py helpers: get_user_tz, user_today, user_briefing_date
  (the briefing day flips at 4am local to align with the compilation slot,
  so the 00:00-04:00 local window still shows yesterday's briefing until
  the new one is generated).
- calendar create/list/update tools now parse naive datetimes in the
  user's TZ before converting to UTC for storage, and tool descriptions
  tell the model to pass plain local dates.
- briefing_conversations.get_or_create_today_conversation and the
  reset-today route use user_briefing_date so the in-progress briefing
  doesn't get replaced at 19:00 NY / UTC midnight.
- _run_profile_closeout targets user-local "yesterday" for consistency.

Regression tests added for the TZ helpers and the calendar tool.

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

src/fabledassistant/services/briefing_scheduler.py

@@ -445,9 +445,12 @@ async def _run_profile_closeout(user_id: int, model: str) -> None:
     """
     from fabledassistant.services.user_profile import append_observations
     from fabledassistant.services.llm import generate_completion
+    from fabledassistant.services.tz import user_today
     from fabledassistant.models.conversation import Conversation, Message
 
-    yesterday = (date.today() - timedelta(days=1))
+    # User-local "yesterday" so closeout always targets the day that just
+    # ended in the user's timezone, regardless of container TZ.
+    yesterday = (await user_today(user_id)) - timedelta(days=1)
     async with async_session() as session:
         result = await session.execute(
             select(Conversation).where(