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

frontend/components/shared/Sidebar.tsx

@@ -8,6 +8,7 @@ import {
   Users,
   Tags,
   Heart,
+  Bell,
   Settings,
   Landmark,
   LogOut,
@@ -24,6 +25,7 @@ const NAV = [
   { href: "/members", label: "Members", icon: Users, adminOnly: false },
   { href: "/topics", label: "Topics", icon: Tags, adminOnly: false },
   { href: "/following", label: "Following", icon: Heart, adminOnly: false },
+  { href: "/notifications", label: "Notifications", icon: Bell, adminOnly: false },
   { href: "/settings", label: "Admin", icon: Settings, adminOnly: true },
 ];