fix: SMS 验证码使用纯数字，兼容 UniSMS 通用模板

UniSMS 通用模板要求 code 为纯数字，原来用 UUID 截取会包含字母。
新增 GenerateNumericCode 函数，SMS 验证码专用。

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

common/verification.go

@@ -1,6 +1,7 @@
 package common
 
 import (
+	"math/rand"
 	"strings"
 	"sync"
 	"time"
@@ -33,6 +34,14 @@ func GenerateVerificationCode(length int) string {
 	return code[:length]
 }
 
+func GenerateNumericCode(length int) string {
+	digits := make([]byte, length)
+	for i := range digits {
+		digits[i] = '0' + byte(rand.Intn(10))
+	}
+	return string(digits)
+}
+
 func RegisterVerificationCodeWithKey(key string, code string, purpose string) {
 	verificationMutex.Lock()
 	defer verificationMutex.Unlock()

controller/misc.go

@@ -319,7 +319,7 @@ func SendSmsVerification(c *gin.Context) {
 		common.ApiErrorI18n(c, i18n.MsgUserPhoneAlreadyTaken)
 		return
 	}
-	code := common.GenerateVerificationCode(6)
+	code := common.GenerateNumericCode(6)
 	common.RegisterVerificationCodeWithKey(phone, code, common.SmsVerificationPurpose)
 	err := common.SendSMS(phone, code)
 	if err != nil {