feat: add email verification code for registration and optimize email service

- Add POST /user/email/code endpoint to send 6-digit verification code
- Require email code verification before completing registration
- Add email code cache with 10min TTL and 5/day send rate limit
- Fix nil client guard, TLS conn leak, domain parsing, and Resend error body in email pkg
- Deploy via ssh inline command using current branch

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

api/router.go

@@ -43,6 +43,7 @@ func SetupRouter(engine *gin.RouterGroup) {
 		userRouter := v1.Group("/user")
 		{
 			userRouter.POST("/sms", userEndpoint.SendVerificationCode)
+			userRouter.POST("/email/code", userEndpoint.SendEmailVerificationCode)
 			userRouter.POST("/register", userEndpoint.RegisterByEmail)
 			userRouter.POST("/login", userEndpoint.LoginByEmail)
 			userRouter.GET("/oauth/google/url", userEndpoint.GetGoogleOAuthUrl)