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_conversations.py

@@ -1,12 +1,13 @@
 """Create and manage briefing conversations."""
 
 import logging
-from datetime import date, datetime, timezone
+from datetime import datetime, timezone
 
 from sqlalchemy import select
 
 from fabledassistant.models import async_session
 from fabledassistant.models.conversation import Conversation, Message
+from fabledassistant.services.tz import user_briefing_date
 
 logger = logging.getLogger(__name__)
 
@@ -15,7 +16,10 @@ async def get_or_create_today_conversation(user_id: int, model: str) -> Conversa
     """
     Return today's briefing conversation, creating it if it doesn't exist.
     """
-    today = date.today()
+    # "Today" is the user-local briefing day (flips at 4am local), not
+    # ``date.today()`` — in a UTC container the latter rolls over at
+    # 19:00 NY local and makes the in-progress briefing disappear.
+    today = await user_briefing_date(user_id)
     async with async_session() as session:
         result = await session.execute(
             select(Conversation).where(