fix: 修复V15迁移脚本MySQL语法错误，移除不支持的IF NOT EXISTS语法

par-core/src/main/resources/db/migration/V15__risk_rules_site_id.sql

@@ -1,2 +1,7 @@
-ALTER TABLE risk_rules ADD COLUMN IF NOT EXISTS site_id VARCHAR(255) DEFAULT NULL AFTER site_type;
+SET @exist := (SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS WHERE table_schema = DATABASE() AND table_name = 'risk_rules' AND column_name = 'site_id');
+SET @sqlstmt := IF(@exist = 0, 'ALTER TABLE risk_rules ADD COLUMN site_id VARCHAR(255) DEFAULT NULL AFTER site_type', 'SELECT ''Column site_id already exists''');
+PREPARE stmt FROM @sqlstmt;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
 CREATE INDEX IF NOT EXISTS idx_risk_rules_site_id ON risk_rules(site_id);