test(config-backup): pin Back up now against a custom folder that does not answer

Merge-time fix for #661. The unreachable-folder table covered GET, diff and
restore; POST /api/config-backups (Back up now) now has its row too.
runConfigBackup already probes before it touches the folder, and the row
fails when the temp sweep is moved ahead of the probe.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/routes/config-backup-unreachable.test.ts

@@ -1,6 +1,7 @@
 /**
  * @fileoverview A custom backup folder that stops answering (a network mount) must never be listed or
- * read before the bounded probe says it is there: GET, diff and restore answer 503 without touching it.
+ * read before the bounded probe says it is there: GET, Back up now, diff and restore answer 503 without
+ * touching it.
  * With only `configBackupDir` in settings.json the recorded-retention fallback would otherwise
  * `readdirSync` the folder first (found in review of #661).
  *
@@ -65,6 +66,7 @@ afterEach(() => {
 describe('a custom backup folder that does not answer', () => {
   it.each([
     ['GET', '/api/config-backups'],
+    ['POST', '/api/config-backups'],
     ['GET', '/api/config-backups/diff?from=cfg-20261001-120000'],
     ['POST', '/api/config-backups/cfg-20261001-120000/restore'],
   ] as const)('%s %s answers 503 without reading the folder', async (method, url) => {