refactor: revive cleanup — unused params to _, add missing doc comments

- admin/db_helper GetCache/GetUserService/GetAuthService: ctx -> _ (签名对称保留)
- validateMergedStorageConfig / ParseMigrationTargetConfig / MockStorageService.Put 未用参数 -> _
- SetDBServiceForTest、StorageDriver 常量组补充文档注释
lint_issues 33→26

backend/plugins/domain/message_gateway/db_helper.go

@@ -21,6 +21,7 @@ var (
 	taskSvc  contracts.TaskService
 )
 
+// SetDBServiceForTest injects a DBService for tests. Production wiring must use Apply.
 func SetDBServiceForTest(s contracts.DBService) {
 	setDBService(s)
 }