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/main.py

@@ -1,7 +1,7 @@
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
-from app.api import bills, members, follows, dashboard, search, settings, admin, health, auth
+from app.api import bills, members, follows, dashboard, search, settings, admin, health, auth, notifications
 from app.config import settings as config
 
 app = FastAPI(
@@ -27,3 +27,4 @@ app.include_router(search.router, prefix="/api/search", tags=["search"])
 app.include_router(settings.router, prefix="/api/settings", tags=["settings"])
 app.include_router(admin.router, prefix="/api/admin", tags=["admin"])
 app.include_router(health.router, prefix="/api/health", tags=["health"])
+app.include_router(notifications.router, prefix="/api/notifications", tags=["notifications"])