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/app/bills/[id]/page.tsx

@@ -4,12 +4,12 @@ import { use } from "react";
 import Link from "next/link";
 import { ArrowLeft, ExternalLink, User } from "lucide-react";
 import { useBill, useBillTrend } from "@/lib/hooks/useBills";
-import { AIBriefCard } from "@/components/bills/AIBriefCard";
+import { BriefPanel } from "@/components/bills/BriefPanel";
 import { ActionTimeline } from "@/components/bills/ActionTimeline";
 import { TrendChart } from "@/components/bills/TrendChart";
 import { NewsPanel } from "@/components/bills/NewsPanel";
 import { FollowButton } from "@/components/shared/FollowButton";
-import { billLabel, formatDate, partyBadgeColor, cn } from "@/lib/utils";
+import { billLabel, congressLabel, formatDate, partyBadgeColor, cn } from "@/lib/utils";
 
 export default function BillDetailPage({ params }: { params: Promise<{ id: string }> }) {
   const { id } = use(params);
@@ -46,7 +46,7 @@ export default function BillDetailPage({ params }: { params: Promise<{ id: strin
               {label}
             </span>
             <span className="text-sm text-muted-foreground">{bill.chamber}</span>
-            <span className="text-sm text-muted-foreground">119th Congress</span>
+            <span className="text-sm text-muted-foreground">{congressLabel(bill.congress_number)}</span>
           </div>
           <h1 className="text-xl font-bold leading-snug">
             {bill.short_title || bill.title || "Untitled Bill"}
@@ -80,7 +80,7 @@ export default function BillDetailPage({ params }: { params: Promise<{ id: strin
       {/* Content grid */}
       <div className="grid grid-cols-1 md:grid-cols-3 gap-4 md:gap-6">
         <div className="md:col-span-2 space-y-6">
-          <AIBriefCard brief={bill.latest_brief} />
+          <BriefPanel briefs={bill.briefs} />
           <ActionTimeline actions={bill.actions} />
         </div>
         <div className="space-y-4">