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/components/bills/AIBriefCard.tsx

@@ -31,6 +31,14 @@ function CitedItem({ point, icon, govinfo_url, openKey, activeKey, setActiveKey
       <div className="flex items-start gap-2">
         <span className="mt-0.5 shrink-0">{icon}</span>
         <span className="flex-1">{cited ? point.text : point}</span>
+        {cited && point.label === "inference" && (
+          <span
+            title="This point is an analytical interpretation, not a literal statement from the bill text"
+            className="shrink-0 text-[10px] px-1.5 py-0.5 rounded border border-border text-muted-foreground font-sans leading-none"
+          >
+            Inferred
+          </span>
+        )}
         {cited && (
           <button
             onClick={() => setActiveKey(isOpen ? null : openKey)}