fix: 移除timestamp字段的显式default声明

DDL生成器会自动为timestamp类型添加DEFAULT CURRENT_TIMESTAMP，
显式指定会导致重复DEFAULT声明，MySQL报错1067。

修复的表：
- discount_marketing (created_at, updated_at)
- discount_promo_code (created_at)
- discount_customer_bind (created_at)

models/discount_customer_bind.json

@@ -54,7 +54,6 @@
             "name": "created_at",
             "title": "绑定时间",
             "type": "timestamp",
-            "default": "CURRENT_TIMESTAMP",
             "nullable": "no"
         }
     ],

models/discount_marketing.json

@@ -56,14 +56,12 @@
             "name": "created_at",
             "title": "创建时间",
             "type": "timestamp",
-            "default": "CURRENT_TIMESTAMP",
             "nullable": "no"
         },
         {
             "name": "updated_at",
             "title": "更新时间",
             "type": "timestamp",
-            "default": "CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP",
             "nullable": "no"
         }
     ],

models/discount_promo_code.json

@@ -54,7 +54,6 @@
             "name": "created_at",
             "title": "生成时间",
             "type": "timestamp",
-            "default": "CURRENT_TIMESTAMP",
             "nullable": "no"
         }
     ],