feat: per-user timezone-aware briefing scheduler

Briefing slots (4am/8am/12pm/4pm) now fire in each user's local timezone
rather than UTC, so the schedule matches the user's actual day.

Backend:
- briefing_scheduler: replaced 4 global UTC cron jobs with per-user
  CronTrigger jobs keyed to the user's IANA timezone; update_user_schedule()
  live-patches the scheduler when config is saved (no restart needed)
- Catchup logic evaluates missed slots in the user's local timezone
- put_config route calls update_user_schedule() after saving

Frontend:
- New Timezone section in Briefing settings: text input pre-filled from
  browser (Intl.DateTimeFormat) with a Detect button for re-detection
- Slot times shown as fixed local times (4:00 am etc.) with the configured
  timezone displayed beneath, replacing the old UTC-conversion display
- timezone field added to BriefingConfig type and default

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

src/fabledassistant/routes/briefing.py

@@ -44,6 +44,9 @@ async def get_config():
 async def put_config():
     data = await request.get_json()
     await set_settings_batch(g.user.id, {"briefing_config": json.dumps(data)})
+    # Live-patch the scheduler so the new timezone takes effect immediately.
+    from fabledassistant.services.briefing_scheduler import update_user_schedule
+    update_user_schedule(g.user.id, data)
     return jsonify({"ok": True})