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/models/news.py

@@ -1,4 +1,4 @@
-from sqlalchemy import Column, Integer, String, Text, Float, DateTime, ForeignKey, Index
+from sqlalchemy import Column, Integer, String, Text, Float, DateTime, ForeignKey, Index, UniqueConstraint
 from sqlalchemy.orm import relationship
 from sqlalchemy.sql import func
 
@@ -12,7 +12,7 @@ class NewsArticle(Base):
     bill_id = Column(String, ForeignKey("bills.bill_id", ondelete="CASCADE"), nullable=False)
     source = Column(String(200))
     headline = Column(Text)
-    url = Column(String, unique=True)
+    url = Column(String)
     published_at = Column(DateTime(timezone=True))
     relevance_score = Column(Float, default=0.0)
     created_at = Column(DateTime(timezone=True), server_default=func.now())
@@ -20,6 +20,7 @@ class NewsArticle(Base):
     bill = relationship("Bill", back_populates="news_articles")
 
     __table_args__ = (
+        UniqueConstraint("bill_id", "url", name="uq_news_articles_bill_url"),
         Index("ix_news_articles_bill_id", "bill_id"),
         Index("ix_news_articles_published_at", "published_at"),
     )