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/0012_dedupe_bill_actions_unique.py

@@ -0,0 +1,32 @@
+"""Deduplicate bill_actions and add unique constraint on (bill_id, action_date, action_text)
+
+Revision ID: 0012
+Revises: 0011
+"""
+from alembic import op
+
+revision = "0012"
+down_revision = "0011"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Remove duplicate rows keeping the lowest id for each (bill_id, action_date, action_text)
+    op.execute("""
+        DELETE FROM bill_actions a
+        USING bill_actions b
+        WHERE a.id > b.id
+          AND a.bill_id = b.bill_id
+          AND a.action_date IS NOT DISTINCT FROM b.action_date
+          AND a.action_text IS NOT DISTINCT FROM b.action_text
+    """)
+    op.create_unique_constraint(
+        "uq_bill_actions_bill_date_text",
+        "bill_actions",
+        ["bill_id", "action_date", "action_text"],
+    )
+
+
+def downgrade():
+    op.drop_constraint("uq_bill_actions_bill_date_text", "bill_actions", type_="unique")