Backup: probe the target with a real write instead of access(W_OK)

On the server the backup folder is a Synology NFS share with "map all
users to admin": access(W_OK) returns EACCES for uid 1000 although
writes succeed, so the first scheduled run failed with "not usable".
Create and remove a probe file instead; test that it leaves nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/backup.md

@@ -14,7 +14,9 @@ holds it). Implementation: [`backup.js`](../backup.js); tests: `test/backup.test
 
 The folder must already exist and be writable by uid 1000 (`create_host_path: false`:
 a missing folder makes `docker compose up` fail instead of silently creating an empty
-one on the wrong disk). Use another disk or a NAS share, never a folder inside `DATA`.
+one on the wrong disk). Use another disk or a NAS share, never a folder inside `DATA`. Writability is checked by
+creating and removing a probe file (`.circus-backup-probe-<pid>`), not with `access()`, which
+NFS shares with squashed ids (e.g. Synology "map all users to admin") answer wrongly.
 Without `BACKUP_PATH`, Compose still mounts the unused default `./backup`, so that
 folder must exist too (`node tools/prepare-compose.js` creates it). Upgrading an
 existing deployment therefore needs either `BACKUP_PATH` in `.env` or an empty