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>

internal/model/user/user.go

@@ -23,9 +23,16 @@ type UserInfoResponse struct {
 	Email    string `json:"email"`
 }
 
+type EmailCodeSendRequest struct {
+	Email string `json:"email" binding:"required,email"`
+}
+
+type EmailCodeSendResponse struct{}
+
 type EmailRegisterRequest struct {
 	Email    string `json:"email" binding:"required,email"`
 	Password string `json:"password" binding:"required,min=6"`
+	Code     string `json:"code" binding:"required"`
 }
 
 type EmailRegisterResponse struct {