fix(v2): webhook 截断 UTF-8 安全 + 投递门禁查库失败落日志(与未付区分)

- MarkFailed: truncateUTF8 避免截断中点 UTF-8 rune, last_error 安全 <=255B
- notifier truncate: 同上, 投递失败消息截断 UTF-8 安全
- deliverOne: 拆分 orderPaid 错误分支 — DB 错误落日志 [webhook] 投递门禁查单失败,与"订单未付"(静默)区分
- test: MarkFailed with long Chinese string, 验证 stored last_error 为有效 UTF-8 且 <=255B

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

internal/store/webhook.go

@@ -2,6 +2,7 @@ package store
 
 import (
 	"fmt"
+	"unicode/utf8"
 
 	"gorm.io/gorm"
 	"gorm.io/gorm/clause"
@@ -51,12 +52,22 @@ func (s *WebhookStore) MarkDelivered(id uint64) error {
 	return nil
 }
 
+// truncateUTF8 safely truncates a string to n bytes without splitting UTF-8 runes.
+func truncateUTF8(s string, n int) string {
+	if len(s) <= n {
+		return s
+	}
+	s = s[:n]
+	for len(s) > 0 && !utf8.ValidString(s) {
+		s = s[:len(s)-1]
+	}
+	return s
+}
+
 // MarkFailed increments attempts and records the last error, leaving the row
 // undelivered for the next retry sweep.
 func (s *WebhookStore) MarkFailed(id uint64, errMsg string) error {
-	if len(errMsg) > 255 {
-		errMsg = errMsg[:255]
-	}
+	errMsg = truncateUTF8(errMsg, 255)
 	if err := s.db.Model(&model.WebhookDelivery{}).Where("id = ?", id).
 		Updates(map[string]any{
 			"attempts":   gorm.Expr("attempts + 1"),