feat: multi-user UI login with admin and monitor roles

Add sac_users table, JWT roles, settings/users API guarded for admin, Users page in UI, and sac_manage_user.py CLI.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/models/__init__.py

@@ -5,6 +5,7 @@ from app.models.notification_channel import NotificationChannel
 from app.models.notification_cooldown import NotificationCooldown
 from app.models.notification_policy import NotificationPolicy
 from app.models.problem import Problem, ProblemEvent
+from app.models.user import User
 
 __all__ = [
     "ApiKey",
@@ -15,4 +16,5 @@ __all__ = [
     "NotificationPolicy",
     "Problem",
     "ProblemEvent",
+    "User",
 ]