feat: personal notes on bill detail pages

- bill_notes table (migration 0014): user_id, bill_id, content, pinned,
  created_at, updated_at; unique constraint (user_id, bill_id)
- BillNote SQLAlchemy model with back-refs on User and Bill
- GET/PUT/DELETE /api/notes/{bill_id} — auth-required, one note per (user, bill)
- NotesPanel component: collapsible, auto-resize textarea, pin toggle,
  save + delete; shows last-saved date and pin indicator in collapsed header
- Pinned notes render above BriefPanel; unpinned render below DraftLetterPanel
- Guests see nothing (token guard in component + query disabled)

Co-Authored-By: Jack Levy

backend/app/models/bill.py

@@ -39,6 +39,7 @@ class Bill(Base):
     news_articles = relationship("NewsArticle", back_populates="bill", order_by="desc(NewsArticle.published_at)")
     trend_scores = relationship("TrendScore", back_populates="bill", order_by="desc(TrendScore.score_date)")
     committee_bills = relationship("CommitteeBill", back_populates="bill")
+    notes = relationship("BillNote", back_populates="bill", cascade="all, delete-orphan")
 
     __table_args__ = (
         Index("ix_bills_congress_number", "congress_number"),