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/congress_poller.py

@@ -338,12 +338,29 @@ def _update_bill_if_changed(db, existing: Bill, parsed: dict) -> bool:
         from app.workers.notification_utils import (
             emit_bill_notification,
             emit_member_follow_notifications,
+            emit_topic_follow_notifications,
             is_milestone_action,
+            is_referral_action,
         )
-        if is_milestone_action(parsed.get("latest_action_text", "")):
-            action_text = parsed["latest_action_text"]
-            emit_bill_notification(db, existing, "bill_updated", action_text)
-            emit_member_follow_notifications(db, existing, "bill_updated", action_text)
+        action_text = parsed.get("latest_action_text", "")
+        is_milestone = is_milestone_action(action_text)
+        is_referral = not is_milestone and is_referral_action(action_text)
+        if is_milestone or is_referral:
+            tier = "progress" if is_milestone else "referral"
+            emit_bill_notification(db, existing, "bill_updated", action_text, milestone_tier=tier)
+            emit_member_follow_notifications(db, existing, "bill_updated", action_text, milestone_tier=tier)
+            # Topic followers — pull tags from the bill's latest brief
+            from app.models.brief import BillBrief
+            latest_brief = (
+                db.query(BillBrief)
+                .filter_by(bill_id=existing.bill_id)
+                .order_by(BillBrief.created_at.desc())
+                .first()
+            )
+            topic_tags = latest_brief.topic_tags or [] if latest_brief else []
+            emit_topic_follow_notifications(
+                db, existing, "bill_updated", action_text, topic_tags, milestone_tier=tier
+            )
     return changed