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/upload/shared/test_helpers.go

@@ -120,7 +120,7 @@ func NewMockStorageService() *MockStorageService {
 }
 
 // Put uploads an object into mock storage.
-func (m *MockStorageService) Put(_ context.Context, key string, body io.Reader, size int64, contentType string) (contracts.StoragePutResult, error) {
+func (m *MockStorageService) Put(_ context.Context, key string, body io.Reader, _ int64, contentType string) (contracts.StoragePutResult, error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	data, err := io.ReadAll(body)

backend/plugins/domain/upload/storage/migration.go

@@ -36,7 +36,7 @@ func LatestMigrationExecution(ctx context.Context) (*contracts.TaskExecutionDTO,
 }
 
 // ParseMigrationTargetConfig parses and validates a storage migration target payload.
-func ParseMigrationTargetConfig(ctx context.Context, payload []byte) (contracts.StorageConfigDTO, error) {
+func ParseMigrationTargetConfig(_ context.Context, payload []byte) (contracts.StorageConfigDTO, error) {
 	if strings.TrimSpace(string(payload)) == "" {
 		return contracts.StorageConfigDTO{}, errors.New("storage migration target payload is required")
 	}