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

frontend/lib/types.ts

@@ -10,15 +10,23 @@ export interface Member {
   photo_url?: string;
 }
 
+export interface CitedPoint {
+  text: string;
+  citation: string;
+  quote: string;
+}
+
 export interface BriefSchema {
   id: number;
+  brief_type?: string;
   summary?: string;
-  key_points?: string[];
-  risks?: string[];
+  key_points?: (string | CitedPoint)[];
+  risks?: (string | CitedPoint)[];
   deadlines?: { date: string | null; description: string }[];
   topic_tags?: string[];
   llm_provider?: string;
   llm_model?: string;
+  govinfo_url?: string;
   created_at?: string;
 }