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

PocketVeto — Feature Roadmap.md

@@ -54,7 +54,7 @@
 - [ ] **Collections / Watchlists** — `collections` (id, user_id, name, slug, is_public) + `collection_bills` join table. UI to create/manage collections and filter dashboard by collection. Shareable via public slug URL (read-only for non-owners).
 - [ ] **Personal Notes** — `bill_notes` table (user_id, bill_id, content, stance, tags, pinned). Shown on bill detail page. Private; optionally pin to top of the bill detail view.
 - [ ] **Shareable Links** — UUID token on briefs and collections → public read-only view, no login required. Same token system for both. No expiry by default. UUID (not sequential) to prevent enumeration.
-- [ ] **Weekly Digest** — Celery beat task (weekly), queries followed bills for changes in the past 7 days, formats a low-noise summary, dispatches via ntfy + RSS.
+- [x] **Weekly Digest** — Celery beat task (Monday 8:30 AM UTC), queries followed bills for changes in the past 7 days, formats a low-noise summary, dispatches via ntfy + RSS. Admin can trigger immediately from Manual Controls.
 
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