fix: make rclone sync more resilient to Scaleway throttling

Offsite sync hit 17 fatal IO errors deleting stale segment files after a
large one-off prune batch (from the recent log/-exclude change), and
rclone's default behavior is to bail immediately on delete errors rather
than retry. Lower concurrency (--transfers 8->4, --checkers 16->8) to
reduce rate-limit pressure, and make retries explicit (--retries 5,
--retries-sleep 10s, --low-level-retries 20) instead of relying on
defaults.

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

borg-backup.sh

@@ -298,8 +298,11 @@ step "Step 6: Syncing to ${RCLONE_REMOTE}:${RCLONE_PATH}"
 run_cmd timeout "$SYNC_TIMEOUT" \
     rclone sync -v \
         --fast-list \
-        --transfers=8 \
-        --checkers=16 \
+        --transfers=4 \
+        --checkers=8 \
+        --retries 5 \
+        --retries-sleep 10s \
+        --low-level-retries 20 \
         --s3-no-check-bucket \
         --max-delete "$RCLONE_MAX_DELETE" \
         "$REPO" "${RCLONE_REMOTE}:${RCLONE_PATH}"