✅ test(backup): fix type error in backup utility tests

- add type assertion to readdir mock return value to resolve TypeScript type mismatch
- ensure proper type handling for file system mock responses in backup tests

src/server/utils/__tests__/backup.test.ts

@@ -154,7 +154,7 @@ describe('DatabaseBackup', () => {
       const oldFileTime = now - (8 * 24 * 60 * 60 * 1000) // 8天前
       const newFileTime = now - (6 * 24 * 60 * 60 * 1000) // 6天前
 
-      vi.mocked(fs.promises.readdir).mockResolvedValue(['backup-old.dump', 'backup-new.dump'])
+      vi.mocked(fs.promises.readdir).mockResolvedValue(['backup-old.dump', 'backup-new.dump'] as any)
       vi.mocked(fs.promises.stat)
         .mockResolvedValueOnce({ mtimeMs: oldFileTime } as any)
         .mockResolvedValueOnce({ mtimeMs: newFileTime } as any)