feat: bill action pipeline, What Changed UI, citation backfill, admin panel

Backend:
- Add fetch_bill_actions task with pagination and idempotent upsert
- Add fetch_actions_for_active_bills nightly batch (4 AM UTC beat schedule)
- Wire fetch_bill_actions into new-bill creation and _update_bill_if_changed
- Add backfill_brief_citations task: detects pre-citation briefs by JSONB
  type check, deletes them, re-queues LLM processing against stored text
  (LLM calls only — zero Congress.gov or GovInfo calls)
- Add admin endpoints: POST /bills/{id}/reprocess, /backfill-citations,
  /trigger-fetch-actions; add uncited_briefs count to /stats

Frontend:
- New BriefPanel component: wraps AIBriefCard, adds amber "What Changed"
  badge for amendment briefs and collapsible version history with
  inline brief expansion
- Swap AIBriefCard for BriefPanel on bill detail page
- Admin panel: Backfill Citations + Fetch Bill Actions buttons; amber
  warning in stats when uncited briefs remain
- Add feature roadmap document with phased plan through Phase 5

Co-Authored-By: Jack Levy

backend/app/workers/celery_app.py

@@ -68,5 +68,9 @@ celery_app.conf.update(
             "task": "app.workers.member_interest.calculate_all_member_trend_scores",
             "schedule": crontab(hour=3, minute=0),
         },
+        "fetch-actions-active-bills": {
+            "task": "app.workers.congress_poller.fetch_actions_for_active_bills",
+            "schedule": crontab(hour=4, minute=0),  # 4 AM UTC, after trend + member scoring
+        },
     },
 )