The prometheus-backup sidecar runs monthly on the 1st SUNDAY 04:00 UTC.
Consecutive first-Sundays can be ~35 days apart (e.g. May 3 -> Jun 7), but
the alert threshold was 32d (2764800s) -> it false-fired every year for the
~3 days between day-32 and the next run. Raised to 40d (3456000s): clears
the max first-Sunday spacing with margin, still catches a genuinely missed
monthly backup. Backup itself is healthy (last May 3, next Jun 7). Verified:
live rule now > 3.456e6, alert state inactive.
63ee655c08