fix(backup): 修复差异/清单设计评审发现的三处问题 (#92)

1) DeleteRecord 拒绝删除仍被成功差异依赖的全量（+CountDependentDifferentials），堵住手动删除孤立差异链的数据完整性缺口；2) 列表查询 Omit(Manifest)，差异基线改按需 FindByID 加载，避免清单大列拖累热路径；3) FileRunner.Run 每文件即时关闭句柄，杜绝大目录 FD 泄漏。含仓储层单测。

server/internal/backup/file_runner.go

@@ -128,13 +128,19 @@ func (r *FileRunner) Run(_ context.Context, task TaskSpec, writer LogWriter) (*R
 			}
 
 			if currentInfo.Mode().IsRegular() {
-				file, err := os.Open(currentPath)
-				if err != nil {
-					return err
-				}
-				defer file.Close()
-				if _, err := io.CopyN(tw, file, currentInfo.Size()); err != nil && err != io.EOF {
-					return err
+				// 每个文件在独立作用域内打开并关闭，避免在大目录树中累积打开的文件句柄。
+				if copyErr := func() error {
+					file, err := os.Open(currentPath)
+					if err != nil {
+						return err
+					}
+					defer file.Close()
+					if _, err := io.CopyN(tw, file, currentInfo.Size()); err != nil && err != io.EOF {
+						return err
+					}
+					return nil
+				}(); copyErr != nil {
+					return copyErr
 				}
 				fileCount++
 				if fileCount%100 == 0 {

server/internal/backup/retention/service_test.go

@@ -45,6 +45,9 @@ func (r *fakeRecordRepository) ListByTask(_ context.Context, _ uint) ([]model.Ba
 func (r *fakeRecordRepository) ListSuccessfulByTask(_ context.Context, _ uint) ([]model.BackupRecord, error) {
 	return r.records, nil
 }
+func (r *fakeRecordRepository) CountDependentDifferentials(context.Context, uint) (int64, error) {
+	return 0, nil
+}
 func (r *fakeRecordRepository) Count(context.Context) (int64, error)                 { return 0, nil }
 func (r *fakeRecordRepository) CountSince(context.Context, time.Time) (int64, error) { return 0, nil }
 func (r *fakeRecordRepository) CountSuccessSince(context.Context, time.Time) (int64, error) {