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>

pkg/common/errors.go

@@ -18,6 +18,8 @@ const (
 	CodeEmailExists      = 1004
 	CodeEmailNotFound    = 1005
 	CodePasswordMismatch = 1006
+	CodeEmailCodeError   = 1007
+	CodeEmailSendLimit   = 1008
 )
 
 const (
@@ -36,6 +38,8 @@ const (
 	CodeEmailExistsMsg      = "email already registered"
 	CodeEmailNotFoundMsg    = "email not found"
 	CodePasswordMismatchMsg = "password mismatch"
+	CodeEmailCodeErrorMsg   = "email code error"
+	CodeEmailSendLimitMsg   = "email send limit reached"
 )
 
 type BusinessError struct {
@@ -61,4 +65,6 @@ var (
 	ErrEmailExists      = NewError(CodeEmailExists, CodeEmailExistsMsg, nil)
 	ErrEmailNotFound    = NewError(CodeEmailNotFound, CodeEmailNotFoundMsg, nil)
 	ErrPasswordMismatch = NewError(CodePasswordMismatch, CodePasswordMismatchMsg, nil)
+	ErrEmailCodeError   = NewError(CodeEmailCodeError, CodeEmailCodeErrorMsg, nil)
+	ErrEmailSendLimit   = NewError(CodeEmailSendLimit, CodeEmailSendLimitMsg, nil)
 )