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/user.py

@@ -14,6 +14,8 @@ class User(Base):
     hashed_password = Column(String, nullable=False)
     is_admin = Column(Boolean, nullable=False, default=False)
     notification_prefs = Column(JSONB, nullable=False, default=dict)
+    rss_token = Column(String, unique=True, nullable=True, index=True)
     created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     follows = relationship("Follow", back_populates="user", cascade="all, delete-orphan")
+    notification_events = relationship("NotificationEvent", back_populates="user", cascade="all, delete-orphan")