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/__init__.py

@@ -4,6 +4,7 @@ from app.models.follow import Follow
 from app.models.member import Member
 from app.models.member_interest import MemberTrendScore, MemberNewsArticle
 from app.models.news import NewsArticle
+from app.models.note import BillNote
 from app.models.notification import NotificationEvent
 from app.models.setting import AppSetting
 from app.models.trend import TrendScore
@@ -15,6 +16,7 @@ __all__ = [
     "BillAction",
     "BillDocument",
     "BillBrief",
+    "BillNote",
     "Follow",
     "Member",
     "MemberTrendScore",