feat: weekly digest + local-time quiet hours

Weekly Digest (send_weekly_digest Celery task):
- Runs every Monday 8:30 AM UTC via beat schedule
- Queries all followed bills updated in the past 7 days per user
- Sends low-priority ntfy push (Priority: low, Tags: newspaper,calendar)
- Creates a NotificationEvent (weekly_digest type) for RSS feed visibility
- Admin can trigger immediately via POST /api/admin/trigger-weekly-digest
- Manual Controls panel now includes "Send Weekly Digest" button

Local-time quiet hours:
- Browser auto-detects IANA timezone via Intl.DateTimeFormat().resolvedOptions().timeZone
- Timezone saved to notification_prefs alongside quiet_hours_start/end on Save
- Dispatcher converts UTC → user's local time (zoneinfo stdlib) before hour comparison
- Falls back to UTC if timezone absent or unrecognised
- Quiet hours UI: 12-hour AM/PM selectors, shows detected timezone as hint
- Clearing quiet hours also clears stored timezone

Co-Authored-By: Jack Levy

backend/app/api/admin.py

@@ -238,6 +238,14 @@ async def resume_analysis(current_user: User = Depends(get_current_admin)):
     return {"task_id": task.id, "status": "queued"}
 
 
+@router.post("/trigger-weekly-digest")
+async def trigger_weekly_digest(current_user: User = Depends(get_current_admin)):
+    """Send the weekly bill activity summary to all eligible users now."""
+    from app.workers.notification_dispatcher import send_weekly_digest
+    task = send_weekly_digest.delay()
+    return {"task_id": task.id, "status": "queued"}
+
+
 @router.post("/trigger-trend-scores")
 async def trigger_trend_scores(current_user: User = Depends(get_current_admin)):
     from app.workers.trend_scorer import calculate_all_trend_scores