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

frontend/lib/api.ts

@@ -174,6 +174,7 @@ export interface AnalysisStats {
   bills_missing_sponsor: number;
   bills_missing_metadata: number;
   bills_missing_actions: number;
+  unlabeled_briefs: number;
   remaining: number;
 }
 
@@ -236,6 +237,8 @@ export const adminAPI = {
     apiClient.post("/api/admin/backfill-all-actions").then((r) => r.data),
   backfillMetadata: () =>
     apiClient.post("/api/admin/backfill-metadata").then((r) => r.data),
+  backfillLabels: () =>
+    apiClient.post("/api/admin/backfill-labels").then((r) => r.data),
   resumeAnalysis: () =>
     apiClient.post("/api/admin/resume-analysis").then((r) => r.data),
   getApiHealth: () =>