fix(server): sync files to disk

Ensure that all files are flushed after they've been written.

At current, files are not explicitly flushed to disk, which can cause
data corruption. In extreme circumstances, it's possible that uploaded
files may not ever be persisted at all.

server/test/repositories/storage.repository.mock.ts

@@ -72,6 +72,7 @@ export const newStorageRepositoryMock = (): Mocked<RepositoryInterface<StorageRe
     walk: vitest.fn().mockImplementation(async function* () {}),
     rename: vitest.fn(),
     copyFile: vitest.fn(),
+    datasync: vitest.fn(),
     utimes: vitest.fn(),
     watch: vitest.fn().mockImplementation(makeMockWatcher({})),
   };