feat: add dual-engine email service with aliyun smtp and resend routing

Route Chinese domains (edu.cn, qq.com, 163.com, etc.) via Aliyun SMTP
and international addresses via Resend API.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/config.go

@@ -16,6 +16,25 @@ type Config struct {
 	Mathpix   MathpixConfig     `mapstructure:"mathpix"`
 	BaiduOCR  BaiduOCRConfig    `mapstructure:"baidu_ocr"`
 	Google    GoogleOAuthConfig `mapstructure:"google"`
+	Email     EmailConfig       `mapstructure:"email"`
+}
+
+type EmailConfig struct {
+	FromName   string           `mapstructure:"from_name"`
+	FromAddr   string           `mapstructure:"from_addr"`
+	AliyunSMTP AliyunSMTPConfig `mapstructure:"aliyun_smtp"`
+	Resend     ResendEmailConfig `mapstructure:"resend"`
+}
+
+type AliyunSMTPConfig struct {
+	Host     string `mapstructure:"host"`
+	Port     int    `mapstructure:"port"`
+	Username string `mapstructure:"username"`
+	Password string `mapstructure:"password"`
+}
+
+type ResendEmailConfig struct {
+	APIKey string `mapstructure:"api_key"`
 }
 
 type BaiduOCRConfig struct {