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/workers/celery_app.py

@@ -82,5 +82,9 @@ celery_app.conf.update(
             "task": "app.workers.notification_dispatcher.send_notification_digest",
             "schedule": crontab(hour=8, minute=0),  # 8 AM UTC daily
         },
+        "send-weekly-digest": {
+            "task": "app.workers.notification_dispatcher.send_weekly_digest",
+            "schedule": crontab(hour=8, minute=30, day_of_week=1),  # Monday 8:30 AM UTC
+        },
     },
 )