The diagnostic log already rings-buffer to 500 rows, but the audit
log had no cap at all and would grow forever. Adds an opt-in
age-based purge under Settings -> Logs: keep entries for N days,
checked on a configurable interval (hourly through monthly), plus a
manual "Purge now" button. Reuses the existing node-schedule-style
reschedule-on-settings-change pattern from the secret/Tailscale
expiry checkers, but as a plain setInterval since "how often" here is
an interval rather than a specific daily time.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
bobbanandClaude Sonnet 5
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