fix(config): set sensible default values for JSON date format and timezone

Change the default JSON date format from empty to `yyyy-MM-dd HH:mm:ss`
and the default timezone from `GMT` to `GMT+8`, matching the values
previously only configured in the dev profile.

src/main/resources/application.yaml

@@ -23,8 +23,8 @@ spring:
   jackson:
     default-property-inclusion: non_null               # 序列化时忽略 null 字段
     property-naming-strategy: SNAKE_CASE                # 实体驼峰 → JSON snake_case
-    date-format: ${JSON_DATE_FORMAT:}                   # 日期格式，dev 配置中覆盖为 yyyy-MM-dd HH:mm:ss
+    date-format: ${JSON_DATE_FORMAT:yyyy-MM-dd HH:mm:ss}
-    time-zone: ${JSON_TIME_ZONE:GMT}                    # 时区，dev 配置中覆盖为 GMT+8
+    time-zone: ${JSON_TIME_ZONE:GMT+8}
 
 # ---- MyBatis-Plus ----
 mybatis-plus: