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/core/contracts/storage.go

@@ -49,6 +49,7 @@ type IngestResult struct {
 // StorageDriver identifies a supported storage backend.
 type StorageDriver string
 
+// Storage drivers supported by the platform. Values persist in storage configs.
 const (
 	StorageDriverLocal  StorageDriver = "local"
 	StorageDriverS3     StorageDriver = "s3"