Bound and retry complete remote readback checks

etc/app-backup/app-backup.conf

@@ -31,6 +31,13 @@ RCLONE_TIMEOUT="1h"
 RCLONE_CONTIMEOUT="30s"
 # Periodic transfer/check statistics in journald, including otherwise quiet uploads.
 RCLONE_STATS_INTERVAL="1m"
+# Full readback attempts, independent of upload retries. All times are seconds.
+# Deadline includes backend waits; raise it for larger/slower full comparisons.
+RCLONE_CHECK_ATTEMPTS=3
+RCLONE_CHECK_RETRY_DELAY=30
+RCLONE_CHECK_TIMEOUT=300
+RCLONE_CHECK_DEADLINE=3600
+RCLONE_CHECK_LOW_LEVEL_RETRIES=3
 RCLONE_TRANSFERS=2
 RCLONE_CHECKERS=4
 # OneDrive chunks must be multiples of 320 KiB; 80 MiB satisfies this.