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>

tests/test_tz_helpers.py

@@ -0,0 +1,63 @@
+"""Tests for user-timezone helpers (services/tz.py)."""
+
+from datetime import datetime
+from unittest.mock import AsyncMock, patch
+from zoneinfo import ZoneInfo
+
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_get_user_tz_returns_configured_zone():
+    from fabledassistant.services import tz as tz_mod
+
+    with patch.object(tz_mod, "get_setting", AsyncMock(return_value="America/New_York")):
+        zone = await tz_mod.get_user_tz(1)
+    assert zone == ZoneInfo("America/New_York")
+
+
+@pytest.mark.asyncio
+async def test_get_user_tz_falls_back_to_utc_on_bad_input():
+    from fabledassistant.services import tz as tz_mod
+
+    with patch.object(tz_mod, "get_setting", AsyncMock(return_value="Not/AZone")):
+        zone = await tz_mod.get_user_tz(1)
+    assert zone == ZoneInfo("UTC")
+
+
+@pytest.mark.asyncio
+async def test_user_briefing_date_before_4am_returns_yesterday():
+    """00:00–03:59 local still shows yesterday's briefing day."""
+    from fabledassistant.services import tz as tz_mod
+
+    ny = ZoneInfo("America/New_York")
+    # 02:00 NY on 2026-04-13 → briefing day = 2026-04-12
+    fake_now = datetime(2026, 4, 13, 2, 0, tzinfo=ny)
+
+    class _FakeDatetime(datetime):
+        @classmethod
+        def now(cls, tz=None):
+            return fake_now
+
+    with patch.object(tz_mod, "get_user_tz", AsyncMock(return_value=ny)), \
+         patch.object(tz_mod, "datetime", _FakeDatetime):
+        day = await tz_mod.user_briefing_date(1)
+    assert day.isoformat() == "2026-04-12"
+
+
+@pytest.mark.asyncio
+async def test_user_briefing_date_after_4am_returns_today():
+    from fabledassistant.services import tz as tz_mod
+
+    ny = ZoneInfo("America/New_York")
+    fake_now = datetime(2026, 4, 13, 4, 30, tzinfo=ny)
+
+    class _FakeDatetime(datetime):
+        @classmethod
+        def now(cls, tz=None):
+            return fake_now
+
+    with patch.object(tz_mod, "get_user_tz", AsyncMock(return_value=ny)), \
+         patch.object(tz_mod, "datetime", _FakeDatetime):
+        day = await tz_mod.user_briefing_date(1)
+    assert day.isoformat() == "2026-04-13"