fix(ads): add description column only if it does not already exist

initialize/migrate/database/02115_ads.up.sql

@@ -1,2 +1,20 @@
-ALTER TABLE `ads`
-    MODIFY COLUMN `description` VARCHAR(255) DEFAULT '' COMMENT 'Description';
+-- 只有当 ads 表中不存在 description 字段时才添加
+SET
+@col_exists := (
+    SELECT COUNT(*)
+    FROM INFORMATION_SCHEMA.COLUMNS
+    WHERE TABLE_SCHEMA = DATABASE()
+      AND TABLE_NAME = 'ads'
+      AND COLUMN_NAME = 'description'
+);
+
+SET
+@query := IF(
+    @col_exists = 0,
+    'ALTER TABLE `ads` ADD COLUMN `description` VARCHAR(255) DEFAULT '''' COMMENT ''Description'';',
+    'SELECT "Column `description` already exists"'
+);
+
+PREPARE stmt FROM @query;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;