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/models/brief.py

@@ -14,12 +14,13 @@ class BillBrief(Base):
     document_id = Column(Integer, ForeignKey("bill_documents.id", ondelete="SET NULL"), nullable=True)
     brief_type = Column(String(20), nullable=False, server_default="full")  # full | amendment
     summary = Column(Text)
-    key_points = Column(JSONB)      # list[str]
-    risks = Column(JSONB)           # list[str]
+    key_points = Column(JSONB)      # list[{text, citation, quote}]
+    risks = Column(JSONB)           # list[{text, citation, quote}]
     deadlines = Column(JSONB)       # list[{date: str, description: str}]
     topic_tags = Column(JSONB)      # list[str]
     llm_provider = Column(String(50))
     llm_model = Column(String(100))
+    govinfo_url = Column(String, nullable=True)
     created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     bill = relationship("Bill", back_populates="briefs")