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

backend/app/schemas/schemas.py

@@ -200,6 +200,7 @@ class BillSchema(BaseModel):
     latest_brief: Optional[BriefSchema] = None
     latest_trend: Optional[TrendScoreSchema] = None
     updated_at: Optional[datetime] = None
+    has_document: bool = False
 
     model_config = {"from_attributes": True}