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/workers/llm_processor.py

@@ -86,6 +86,7 @@ def process_document_with_llm(self, document_id: int):
             topic_tags=brief.topic_tags,
             llm_provider=brief.llm_provider,
             llm_model=brief.llm_model,
+            govinfo_url=doc.govinfo_url,
         )
         db.add(db_brief)
         db.commit()