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/service/settings_view.go

@@ -43,6 +43,7 @@ type SystemSettings struct {
 
 	DefaultConcurrency int
 	DefaultBalance     float64
+	DefaultSubscriptions []DefaultSubscriptionSetting
 
 	// Model fallback configuration
 	EnableModelFallback      bool   `json:"enable_model_fallback"`
@@ -65,6 +66,11 @@ type SystemSettings struct {
 	MinClaudeCodeVersion string
 }
 
+type DefaultSubscriptionSetting struct {
+	GroupID      int64 `json:"group_id"`
+	ValidityDays int   `json:"validity_days"`
+}
+
 type PublicSettings struct {
 	RegistrationEnabled   bool
 	EmailVerifyEnabled    bool