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/settings/page.tsx

@@ -149,6 +149,11 @@ export default function SettingsPage() {
                   style={{ width: `${pct}%` }}
                 />
               </div>
+              {stats.uncited_briefs > 0 && (
+                <p className="text-xs text-amber-600 dark:text-amber-400">
+                  ⚠ {stats.uncited_briefs.toLocaleString()} brief{stats.uncited_briefs !== 1 ? "s" : ""} missing citations — run Backfill Citations to fix
+                </p>
+              )}
             </div>
           </>
         ) : (
@@ -352,6 +357,18 @@ export default function SettingsPage() {
           >
             <RefreshCw className="w-3.5 h-3.5" /> Backfill Sponsors
           </button>
+          <button
+            onClick={() => trigger("citations", adminAPI.backfillCitations)}
+            className="flex items-center gap-2 px-4 py-2 text-sm bg-amber-100 text-amber-800 hover:bg-amber-200 dark:bg-amber-900/30 dark:text-amber-300 dark:hover:bg-amber-900/50 rounded-md transition-colors"
+          >
+            <RefreshCw className="w-3.5 h-3.5" /> Backfill Citations
+          </button>
+          <button
+            onClick={() => trigger("actions", adminAPI.triggerFetchActions)}
+            className="flex items-center gap-2 px-4 py-2 text-sm bg-muted hover:bg-accent rounded-md transition-colors"
+          >
+            <RefreshCw className="w-3.5 h-3.5" /> Fetch Bill Actions
+          </button>
         </div>
         {Object.entries(taskIds).map(([name, id]) => (
           <p key={name} className="text-xs text-muted-foreground">{name}: task {id} queued</p>