feat(phase2): fact/inference labeling, change-driven alerts, admin cleanup

- Add label: cited_fact | inference to LLM brief schema (all 4 providers)
- Inferred badge in AIBriefCard for inference-labeled points
- backfill_brief_labels Celery task: classifies existing cited points in-place
- POST /api/admin/backfill-labels + unlabeled_briefs stat counter
- Expand milestone keywords: markup, conference
- Add is_referral_action() for committee referrals (referred to)
- Two-tier milestone notifications: progress tier (all follow modes) and
  referral tier (pocket_veto/boost only, neutral suppressed)
- Topic followers now receive bill_updated milestone notifications via
  latest brief topic_tags lookup in _update_bill_if_changed()
- Admin Manual Controls: collapsible Maintenance section for backfill tasks
- Update ARCHITECTURE.md and roadmap for Phase 2 completion

Co-Authored-By: Jack Levy

backend/app/workers/notification_dispatcher.py

@@ -94,6 +94,13 @@ def dispatch_notifications(self):
                 db.commit()
                 continue
 
+            # Referral-tier events (committee referrals) are noisy for neutral follows;
+            # pocket_veto and pocket_boost users want them as early warnings
+            if follow_mode == "neutral" and (event.payload or {}).get("milestone_tier") == "referral":
+                event.dispatched_at = now
+                db.commit()
+                continue
+
             prefs = user.notification_prefs or {}
             ntfy_url = prefs.get("ntfy_topic_url", "").strip()
             ntfy_auth_method = prefs.get("ntfy_auth_method", "none")