fix: 会员支付打印微信完整错误并支持配置 V2 商户密钥

- 统一下单 XML 安全解析，日志输出 raw_response 与 err_code_des
- 下单前打印 club/appid/mch_id/key_source 诊断
- 后台 club-manage 可保存 config_json.mch_key（V2 MD5 密钥，非 api_v3_key）
- 未配 mch_key 时告警：回落全局密钥易导致签名错误

回退: git reset --hard backup-before-wx-v2-diag-20260710

Co-authored-by: Cursor <cursoragent@cursor.com>

jituan/views.py

@@ -443,9 +443,11 @@ class ClubManageView(APIView):
             'sort_order': club.sort_order,
         }
         if full:
+            cfg_json = club.config_json or {}
             base.update({
                 'wx_secret': club.wx_secret,
                 'api_v3_key': club.api_v3_key,
+                'mch_key': cfg_json.get('mch_key') or cfg_json.get('shanghumiyao') or '',
                 'cert_serial_no': club.cert_serial_no,
                 'private_key_path': club.private_key_path,
                 'platform_cert_dir': club.platform_cert_dir,
@@ -494,6 +496,12 @@ class ClubManageView(APIView):
                     if field in request.data:
                         setattr(club, field, request.data[field])
                         update_fields.append(field)
+                if 'mch_key' in request.data:
+                    cfg = dict(club.config_json or {})
+                    cfg['mch_key'] = (request.data.get('mch_key') or '').strip()
+                    club.config_json = cfg
+                    if 'config_json' not in update_fields:
+                        update_fields.append('config_json')
                 if update_fields:
                     club.save(update_fields=update_fields)
                 return Response({