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/api/admin.py

@@ -134,6 +134,23 @@ async def get_stats(
     bills_missing_actions = (await db.execute(
         text("SELECT COUNT(*) FROM bills WHERE actions_fetched_at IS NULL")
     )).scalar()
+    # Cited brief points (objects) that have no label yet
+    unlabeled_briefs = (await db.execute(
+        text("""
+            SELECT COUNT(*) FROM bill_briefs
+            WHERE (
+              key_points IS NOT NULL AND EXISTS (
+                SELECT 1 FROM jsonb_array_elements(key_points) AS p
+                WHERE jsonb_typeof(p) = 'object' AND (p->>'label') IS NULL
+              )
+            ) OR (
+              risks IS NOT NULL AND EXISTS (
+                SELECT 1 FROM jsonb_array_elements(risks) AS r
+                WHERE jsonb_typeof(r) = 'object' AND (r->>'label') IS NULL
+              )
+            )
+        """)
+    )).scalar()
     return {
         "total_bills": total_bills,
         "docs_fetched": docs_fetched,
@@ -146,6 +163,7 @@ async def get_stats(
         "bills_missing_sponsor": bills_missing_sponsor,
         "bills_missing_metadata": bills_missing_metadata,
         "bills_missing_actions": bills_missing_actions,
+        "unlabeled_briefs": unlabeled_briefs,
         "remaining": total_bills - total_briefs,
     }
 
@@ -204,6 +222,14 @@ async def backfill_metadata(current_user: User = Depends(get_current_admin)):
     return {"task_id": task.id, "status": "queued"}
 
 
+@router.post("/backfill-labels")
+async def backfill_labels(current_user: User = Depends(get_current_admin)):
+    """Classify existing cited brief points as fact or inference without re-generating briefs."""
+    from app.workers.llm_processor import backfill_brief_labels
+    task = backfill_brief_labels.delay()
+    return {"task_id": task.id, "status": "queued"}
+
+
 @router.post("/resume-analysis")
 async def resume_analysis(current_user: User = Depends(get_current_admin)):
     """Re-queue LLM processing for docs with no brief, and document fetching for bills with no doc."""