feat(email_gen): draft constituent letter generator + bill text indicators

- Add DraftLetterPanel: collapsible UI below BriefPanel for bills with a
  brief; lets users select up to 3 cited points, pick stance/tone, and
  generate a plain-text letter via the configured LLM provider
- Stance pre-fills from follow mode (pocket_boost → YES, pocket_veto → NO)
  and clears when the user unfollows; recipient derived from bill chamber
- Add POST /api/bills/{bill_id}/draft-letter endpoint with proper LLM
  provider/model resolution from AppSetting (respects Settings page choice)
- Add generate_text() to LLMProvider ABC and all four providers
- Expose has_document on BillSchema (list endpoint) via a single batch
  query; BillCard shows Brief / Pending / No text indicator per bill

Authored-By: Jack Levy

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

@@ -5,6 +5,7 @@ import Link from "next/link";
 import { ArrowLeft, ExternalLink, FileX, User } from "lucide-react";
 import { useBill, useBillNews, useBillTrend } from "@/lib/hooks/useBills";
 import { BriefPanel } from "@/components/bills/BriefPanel";
+import { DraftLetterPanel } from "@/components/bills/DraftLetterPanel";
 import { ActionTimeline } from "@/components/bills/ActionTimeline";
 import { TrendChart } from "@/components/bills/TrendChart";
 import { NewsPanel } from "@/components/bills/NewsPanel";
@@ -104,7 +105,10 @@ export default function BillDetailPage({ params }: { params: Promise<{ id: strin
       <div className="grid grid-cols-1 md:grid-cols-3 gap-4 md:gap-6">
         <div className="md:col-span-2 space-y-6">
           {bill.briefs.length > 0 ? (
-            <BriefPanel briefs={bill.briefs} />
+            <>
+              <BriefPanel briefs={bill.briefs} />
+              <DraftLetterPanel billId={bill.bill_id} brief={bill.briefs[0]} chamber={bill.chamber} />
+            </>
           ) : bill.has_document ? (
             <div className="bg-card border border-border rounded-lg p-6 text-center space-y-2">
               <p className="text-sm font-medium text-muted-foreground">Analysis pending</p>