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>

main.go

@@ -18,6 +18,7 @@ import (
 	"gitea.com/texpixel/document_ai/pkg/cors"
 	"gitea.com/texpixel/document_ai/pkg/log"
 	"gitea.com/texpixel/document_ai/pkg/middleware"
+	"gitea.com/texpixel/document_ai/pkg/email"
 	"gitea.com/texpixel/document_ai/pkg/sms"
 	"github.com/gin-gonic/gin"
 )
@@ -44,6 +45,7 @@ func main() {
 	dao.InitDB(config.GlobalConfig.Database)
 	cache.InitRedisClient(config.GlobalConfig.Redis)
 	sms.InitSmsClient()
+	email.InitEmailClient()
 
 	// 设置gin模式
 	gin.SetMode(config.GlobalConfig.Server.Mode)