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

frontend/lib/api.ts

@@ -135,6 +135,7 @@ export interface AnalysisStats {
   briefs_generated: number;
   full_briefs: number;
   amendment_briefs: number;
+  uncited_briefs: number;
   remaining: number;
 }
 
@@ -159,6 +160,10 @@ export const adminAPI = {
     apiClient.post("/api/admin/trigger-trend-scores").then((r) => r.data),
   backfillSponsors: () =>
     apiClient.post("/api/admin/backfill-sponsors").then((r) => r.data),
+  backfillCitations: () =>
+    apiClient.post("/api/admin/backfill-citations").then((r) => r.data),
+  triggerFetchActions: () =>
+    apiClient.post("/api/admin/trigger-fetch-actions").then((r) => r.data),
   getTaskStatus: (taskId: string) =>
     apiClient.get(`/api/admin/task-status/${taskId}`).then((r) => r.data),
 };