feat(mail): Add support for multiple domains per mail service

- Add `Domains` field to MailServiceConfig for managing additional domains under single API
- Implement `matchDomain` helper function for precise and subdomain matching logic
- Update `GetServiceByDomain` to check both primary domain and additional domains list
- Enhance EmailConfig UI to display domain count and allow comma-separated domain input
- Add domains field to mail service request/response structures in API handlers
- Update frontend types to include domains array in MailService interface
- Improve documentation with clarification on primary vs additional domains usage
- Allows single mail service API to manage multiple email domains for verification and operations

backend/internal/config/config.go

@@ -10,12 +10,13 @@ import (
 
 // MailServiceConfig 邮箱服务配置
 type MailServiceConfig struct {
-	Name       string `yaml:"name" json:"name"`
-	APIBase    string `yaml:"api_base" json:"api_base"`
-	APIToken   string `yaml:"api_token" json:"api_token"`
-	Domain     string `yaml:"domain" json:"domain"`
-	EmailPath  string `yaml:"email_path,omitempty" json:"email_path,omitempty"`
-	AddUserAPI string `yaml:"add_user_api,omitempty" json:"add_user_api,omitempty"`
+	Name       string   `yaml:"name" json:"name"`
+	APIBase    string   `yaml:"api_base" json:"api_base"`
+	APIToken   string   `yaml:"api_token" json:"api_token"`
+	Domain     string   `yaml:"domain" json:"domain"`
+	Domains    []string `yaml:"domains,omitempty" json:"domains,omitempty"` // 附加域名列表（同一个API管理的多个域名）
+	EmailPath  string   `yaml:"email_path,omitempty" json:"email_path,omitempty"`
+	AddUserAPI string   `yaml:"add_user_api,omitempty" json:"add_user_api,omitempty"`
 }
 
 // Config 应用配置 (实时从数据库读取)