fix(news): per-bill URL dedup + lazy re-fetch on bill detail load

- Drop global unique constraint on news_articles.url; replace with
  (bill_id, url) so the same article can appear for multiple bills
- news_fetcher dedup now scoped to bill_id instead of global URL
- Bill detail endpoint triggers a background news fetch when no
  articles are stored, so gnews articles surface on next load

Migration 0009.

Co-Authored-By: Jack Levy

backend/alembic/versions/0009_fix_news_articles_url_uniqueness.py

@@ -0,0 +1,29 @@
+"""fix news_articles url uniqueness to per-bill scope
+
+Previously url was globally unique, meaning the same article could only
+be stored for one bill. This changes it to (bill_id, url) unique so the
+same article can appear in multiple bills' news panels.
+
+Revision ID: 0009
+Revises: 0008
+Create Date: 2026-03-01
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0009"
+down_revision = "0008"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Drop the old global unique constraint on url
+    op.drop_constraint("news_articles_url_key", "news_articles", type_="unique")
+    # Add per-bill unique constraint
+    op.create_unique_constraint("uq_news_articles_bill_url", "news_articles", ["bill_id", "url"])
+
+
+def downgrade():
+    op.drop_constraint("uq_news_articles_bill_url", "news_articles", type_="unique")
+    op.create_unique_constraint("news_articles_url_key", "news_articles", ["url"])