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/user.py

@@ -19,3 +19,4 @@ class User(Base):
 
     follows = relationship("Follow", back_populates="user", cascade="all, delete-orphan")
     notification_events = relationship("NotificationEvent", back_populates="user", cascade="all, delete-orphan")
+    bill_notes = relationship("BillNote", back_populates="user", cascade="all, delete-orphan")