feat(admin): add super admin initialization endpoint

Add POST /api/v1/admin/setup-super-admin endpoint to initialize the first super admin.
The endpoint can only be used once - after a super admin exists, subsequent requests
are rejected. Requires valid setup_secret configured via APP_SETUP_SECRET environment
variable or setup_secret in config file.

internal/errors/app_errors.go

@@ -135,6 +135,11 @@ var (
 	ErrAlreadyVerified            = &AppError{Code: "ALREADY_VERIFIED", Message: "已完成身份认证，无需重复申请"}
 	ErrVerificationAlreadyHandled = &AppError{Code: "VERIFICATION_ALREADY_HANDLED", Message: "该认证申请已被处理"}
 	ErrVerificationRequired       = &AppError{Code: "VERIFICATION_REQUIRED", Message: "需要完成身份认证"}
+
+	// 初始化相关错误
+	ErrSetupAlreadyCompleted    = &AppError{Code: "SETUP_ALREADY_COMPLETED", Message: "超级管理员已初始化，无法重复设置"}
+	ErrInvalidSetupSecret       = &AppError{Code: "INVALID_SETUP_SECRET", Message: "初始化密钥无效"}
+	ErrSetupSecretNotConfigured = &AppError{Code: "SETUP_SECRET_NOT_CONFIGURED", Message: "未配置初始化密钥，无法使用此接口"}
 )
 
 // Wrap 包装错误