Backup directory failures must not crash the whole server

A fresh Windows install reported the server failing around backups.
createBackupService created the backups subdirectory once, synchronously,
at startup with no error handling - any failure there (permissions, a
path beyond Windows' MAX_PATH, a OneDrive/cloud-sync lock, antivirus)
crashed the entire Node process before it started listening, taking
down timing with it instead of just leaving backups unavailable.

Directory creation is now retried lazily and non-fatally on each backup
attempt, and a post-backup file rename retries briefly on EBUSY/EPERM
since a freshly written file is sometimes momentarily locked by
antivirus/cloud-sync scanning on Windows.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-racecore",
-  "version": "1.4.0",
+  "version": "1.4.1",
   "private": true,
   "description": "RC RaceCore live timing and race control with AMMC ingest and SQLite persistence",
   "engines": {