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

frontend/app/settings/page.tsx

@@ -734,6 +734,13 @@ export default function SettingsPage() {
               count: stats?.pending_llm,
               countLabel: "bills pending analysis",
             },
+            {
+              key: "weekly-digest",
+              name: "Send Weekly Digest",
+              description: "Immediately dispatch the weekly bill activity summary to all users who have ntfy or RSS enabled and at least one bill followed. Runs automatically every Monday at 8:30 AM UTC.",
+              fn: adminAPI.triggerWeeklyDigest,
+              status: "on-demand",
+            },
           ];
 
           const maintenance: ControlItem[] = [