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/lib/api.ts

@@ -75,6 +75,15 @@ export const billsAPI = {
     apiClient.get<NewsArticle[]>(`/api/bills/${id}/news`).then((r) => r.data),
   getTrend: (id: string, days?: number) =>
     apiClient.get<TrendScore[]>(`/api/bills/${id}/trend`, { params: { days } }).then((r) => r.data),
+  generateDraft: (id: string, body: {
+    stance: string;
+    recipient: string;
+    tone: string;
+    selected_points: string[];
+    include_citations: boolean;
+    zip_code?: string;
+  }) =>
+    apiClient.post<{ draft: string }>(`/api/bills/${id}/draft-letter`, body).then((r) => r.data),
 };
 
 // Members