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/alembic/versions/0011_add_notifications.py

@@ -0,0 +1,39 @@
+"""add notifications: rss_token on users, notification_events table
+
+Revision ID: 0011
+Revises: 0010
+Create Date: 2026-03-01
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = "0011"
+down_revision = "0010"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column("users", sa.Column("rss_token", sa.String(), nullable=True))
+    op.create_index("ix_users_rss_token", "users", ["rss_token"], unique=True)
+
+    op.create_table(
+        "notification_events",
+        sa.Column("id", sa.Integer(), autoincrement=True, nullable=False),
+        sa.Column("user_id", sa.Integer(), sa.ForeignKey("users.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("bill_id", sa.String(), sa.ForeignKey("bills.bill_id", ondelete="CASCADE"), nullable=False),
+        sa.Column("event_type", sa.String(50), nullable=False),
+        sa.Column("payload", postgresql.JSONB(astext_type=sa.Text()), nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.text("now()")),
+        sa.Column("dispatched_at", sa.DateTime(timezone=True), nullable=True),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index("ix_notification_events_user_id", "notification_events", ["user_id"])
+    op.create_index("ix_notification_events_dispatched_at", "notification_events", ["dispatched_at"])
+
+
+def downgrade():
+    op.drop_table("notification_events")
+    op.drop_index("ix_users_rss_token", table_name="users")
+    op.drop_column("users", "rss_token")