feat(settings): add default subscriptions for new users

- add default subscriptions to admin settings

- auto-assign subscriptions on register and admin user creation

- add validation/tests and align settings UI with subscription selector patterns

backend/internal/server/middleware/jwt_auth_test.go

@@ -40,7 +40,7 @@ func newJWTTestEnv(users map[int64]*service.User) (*gin.Engine, *service.AuthSer
 	cfg.JWT.AccessTokenExpireMinutes = 60
 
 	userRepo := &stubJWTUserRepo{users: users}
-	authSvc := service.NewAuthService(userRepo, nil, nil, cfg, nil, nil, nil, nil, nil)
+	authSvc := service.NewAuthService(userRepo, nil, nil, cfg, nil, nil, nil, nil, nil, nil)
 	userSvc := service.NewUserService(userRepo, nil, nil)
 	mw := NewJWTAuthMiddleware(authSvc, userSvc)