rclone-backup: throttle API calls to survive Drive rate limits

The nightly sync has failed since Jul 18 with RATE_LIMIT_EXCEEDED,
transferring 0 B. Limit transactions and pace Drive requests, and retry
a few times before giving up. This mitigates rather than fixes the cause
(the remote still uses rclone's shared default client_id).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/fileserver.nix

@@ -251,6 +251,9 @@
         ${pkgs.rclone}/bin/rclone sync /data gdrive:backup-daten \
           --fast-list \
           --drive-stop-on-upload-limit \
+          --tpslimit 10 \
+          --drive-pacer-min-sleep 100ms \
+          --retries 3 \
           --log-file=/var/log/rclone-backup.log \
           --log-level=INFO
       '';