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/app/workers/news_fetcher.py

@@ -50,8 +50,8 @@ def fetch_news_for_bill(self, bill_id: str):
             url = article.get("url")
             if not url:
                 continue
-            # Idempotency: skip duplicate URLs
-            existing = db.query(NewsArticle).filter_by(url=url).first()
+            # Idempotency: skip duplicates per bill (same article can appear for multiple bills)
+            existing = db.query(NewsArticle).filter_by(bill_id=bill_id, url=url).first()
             if existing:
                 continue
             pub_at = None