feat: per-user notifications (ntfy + RSS), deduplicated actions, backfill task

Notifications:
- New /notifications page accessible to all users (ntfy + RSS config)
- ntfy now supports no-auth, Bearer token, and HTTP Basic auth (for ACL-protected self-hosted servers)
- RSS enabled/disabled independently of ntfy; token auto-generated on first GET
- Notification settings removed from admin-only Settings page; replaced with link card
- Sidebar adds Notifications nav link for all users
- notification_dispatcher.py: fan-out now marks RSS events dispatched independently

Action history:
- Migration 0012: deduplicates existing bill_actions rows and adds UNIQUE(bill_id, action_date, action_text)
- congress_poller.py: replaces existence-check inserts with ON CONFLICT DO NOTHING (race-condition safe)
- Added backfill_all_bill_actions task (no date filter) + admin endpoint POST /backfill-all-actions

Authored-By: Jack Levy

backend/app/models/__init__.py

@@ -4,6 +4,7 @@ from app.models.follow import Follow
 from app.models.member import Member
 from app.models.member_interest import MemberTrendScore, MemberNewsArticle
 from app.models.news import NewsArticle
+from app.models.notification import NotificationEvent
 from app.models.setting import AppSetting
 from app.models.trend import TrendScore
 from app.models.committee import Committee, CommitteeBill
@@ -19,6 +20,7 @@ __all__ = [
     "MemberTrendScore",
     "MemberNewsArticle",
     "NewsArticle",
+    "NotificationEvent",
     "AppSetting",
     "TrendScore",
     "Committee",