feat: add email verify code endpoint and require code on register

- POST /v1/user/email/code sends a 6-digit verify code via email (rate-limited, 10min TTL)
- RegisterByEmail now validates verify_code before creating the account
- Added email code cache helpers mirroring SMS pattern
- Added error codes 1007 (email code error) and 1008 (send limit)

Co-Authored-By: Claude Sonnet 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.SendEmailVerifyCode)
 			userRouter.POST("/register", userEndpoint.RegisterByEmail)
 			userRouter.POST("/login", userEndpoint.LoginByEmail)
 			userRouter.GET("/oauth/google/url", userEndpoint.GetGoogleOAuthUrl)