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/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) {