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/handler/dto/settings.go

@@ -39,8 +39,9 @@ type SystemSettings struct {
 	PurchaseSubscriptionURL     string `json:"purchase_subscription_url"`
 	SoraClientEnabled           bool   `json:"sora_client_enabled"`
 
-	DefaultConcurrency int     `json:"default_concurrency"`
-	DefaultBalance     float64 `json:"default_balance"`
+	DefaultConcurrency   int     `json:"default_concurrency"`
+	DefaultBalance       float64 `json:"default_balance"`
+	DefaultSubscriptions []DefaultSubscriptionSetting `json:"default_subscriptions"`
 
 	// Model fallback configuration
 	EnableModelFallback      bool   `json:"enable_model_fallback"`
@@ -62,6 +63,11 @@ type SystemSettings struct {
 	MinClaudeCodeVersion string `json:"min_claude_code_version"`
 }
 
+type DefaultSubscriptionSetting struct {
+	GroupID      int64 `json:"group_id"`
+	ValidityDays int   `json:"validity_days"`
+}
+
 type PublicSettings struct {
 	RegistrationEnabled         bool   `json:"registration_enabled"`
 	EmailVerifyEnabled          bool   `json:"email_verify_enabled"`