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/api/bills.py

@@ -109,6 +109,15 @@ async def get_bill(bill_id: str, db: AsyncSession = Depends(get_db)):
         detail.latest_brief = bill.briefs[0]
     if bill.trend_scores:
         detail.latest_trend = bill.trend_scores[0]
+
+    # Trigger a background news refresh if no articles are stored yet
+    if not bill.news_articles:
+        try:
+            from app.workers.news_fetcher import fetch_news_for_bill
+            fetch_news_for_bill.delay(bill_id)
+        except Exception:
+            pass
+
     return detail