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/alembic/versions/0006_add_brief_govinfo_url.py

@@ -0,0 +1,21 @@
+"""add govinfo_url to bill_briefs
+
+Revision ID: 0006
+Revises: 0005
+Create Date: 2026-02-28
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0006"
+down_revision = "0005"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column("bill_briefs", sa.Column("govinfo_url", sa.String(), nullable=True))
+
+
+def downgrade():
+    op.drop_column("bill_briefs", "govinfo_url")