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/tz.py

@@ -0,0 +1,47 @@
+"""User-timezone helpers.
+
+All datetimes in the DB are stored as UTC. The helpers here bridge between
+that UTC storage and the user's configured local timezone (IANA string in
+the ``user_timezone`` setting). Use these anywhere the model or UI talks
+in terms of "today", "tomorrow", or a bare calendar date — never
+``date.today()`` or ``datetime.now(timezone.utc)`` inside a per-user flow.
+"""
+from __future__ import annotations
+
+from datetime import date, datetime, timedelta
+from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
+
+from fabledassistant.services.settings import get_setting
+
+# Briefing day boundary — kept in sync with the compilation slot in
+# ``briefing_scheduler.SLOTS``. The briefing day flips at this local hour
+# (not midnight) so the 00:00–04:00 local window still shows yesterday's
+# briefing until the 4am compilation generates the new one.
+BRIEFING_DAY_START_HOUR = 4
+
+
+async def get_user_tz(user_id: int) -> ZoneInfo:
+    """Return the user's IANA ``ZoneInfo``, falling back to UTC."""
+    tz_str = await get_setting(user_id, "user_timezone") or "UTC"
+    try:
+        return ZoneInfo(tz_str)
+    except (ZoneInfoNotFoundError, KeyError):
+        return ZoneInfo("UTC")
+
+
+async def user_today(user_id: int) -> date:
+    """Return today's calendar date in the user's local timezone."""
+    tz = await get_user_tz(user_id)
+    return datetime.now(tz).date()
+
+
+async def user_briefing_date(user_id: int) -> date:
+    """Return the current "briefing day" in the user's local timezone.
+
+    The briefing day flips at ``BRIEFING_DAY_START_HOUR`` (4am local),
+    aligned with the compilation slot that generates the day's briefing.
+    Between 00:00 and 04:00 local this still returns *yesterday*, so the
+    UI keeps showing the in-progress briefing until the new one is built.
+    """
+    tz = await get_user_tz(user_id)
+    return (datetime.now(tz) - timedelta(hours=BRIEFING_DAY_START_HOUR)).date()