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>

internal/model/user/user.go

@@ -23,9 +23,16 @@ type UserInfoResponse struct {
 	Email    string `json:"email"`
 }
 
+type EmailVerifyCodeRequest struct {
+	Email string `json:"email" binding:"required,email"`
+}
+
+type EmailVerifyCodeResponse struct{}
+
 type EmailRegisterRequest struct {
-	Email    string `json:"email" binding:"required,email"`
-	Password string `json:"password" binding:"required,min=6"`
+	Email      string `json:"email" binding:"required,email"`
+	Password   string `json:"password" binding:"required,min=6"`
+	VerifyCode string `json:"verify_code" binding:"required"`
 }
 
 type EmailRegisterResponse struct {