feat(citations): add per-claim citations to AI briefs

LLM prompts updated to output {text, citation, quote} objects for every
key_point and risk. govinfo_url stored on BillBrief (migration 0006) so
the frontend can link directly to the source document without an extra
query. AIBriefCard renders § citation chips that expand inline to show
the verbatim quote and a View source → GovInfo link. Old plain-string
briefs continue to render unchanged.

Authored-By: Jack Levy

backend/app/schemas/schemas.py

@@ -36,12 +36,13 @@ class BriefSchema(BaseModel):
     id: int
     brief_type: str = "full"
     summary: Optional[str] = None
-    key_points: Optional[list[str]] = None
-    risks: Optional[list[str]] = None
+    key_points: Optional[list[Any]] = None
+    risks: Optional[list[Any]] = None
     deadlines: Optional[list[dict[str, Any]]] = None
     topic_tags: Optional[list[str]] = None
     llm_provider: Optional[str] = None
     llm_model: Optional[str] = None
+    govinfo_url: Optional[str] = None
     created_at: Optional[datetime] = None
 
     model_config = {"from_attributes": True}