支付与收款商户号分离：支付改读 pay_* 独立配置与证书目录，收款侧不动。

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

dingdan/models.py

@@ -64,6 +64,13 @@ class Dingdan(models.Model):
         db_index=True,
         verbose_name='微信支付订单号'
     )
+    pay_mch_id = models.CharField(
+        max_length=32,
+        blank=True,
+        default='',
+        db_index=True,
+        verbose_name='下单支付商户号',
+    )
 
     class Meta:
         db_table = 'dingdan'