fix: explicitly import Site in manager_account to resolve relationship

SQLAlchemy couldn't resolve the string 'Site' in ManagerAccount's
relationship() at query time because Site wasn't guaranteed to be
imported first. Adding the explicit import ensures the mapper is
configured correctly regardless of import order.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cloud_backend/models/manager_account.py

@@ -2,6 +2,7 @@ from sqlalchemy import Column, Integer, String, DateTime, Table, ForeignKey
 from sqlalchemy.orm import relationship
 from sqlalchemy.sql import func
 from database import Base
+from models.site import Site  # noqa: F401 — needed so SQLAlchemy resolves "Site" in relationship
 
 
 # Many-to-many: one manager can access multiple sites, one site can have multiple managers