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/schemas/schemas.py

@@ -4,6 +4,26 @@ from typing import Any, Generic, Optional, TypeVar
 from pydantic import BaseModel
 
 
+# ── Notifications ──────────────────────────────────────────────────────────────
+
+# ── Bill Notes ────────────────────────────────────────────────────────────────
+
+class BillNoteSchema(BaseModel):
+    id: int
+    bill_id: str
+    content: str
+    pinned: bool
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class BillNoteUpsert(BaseModel):
+    content: str
+    pinned: bool = False
+
+
 # ── Notifications ──────────────────────────────────────────────────────────────
 
 class NotificationSettingsResponse(BaseModel):