SiteAuditRetentionService (site-only IHostedService) ticks PurgeExpiredAsync on
InitialDelay(5m)/PurgeInterval(24h, clamped >=1m), cutoff = UtcNow - RetentionDays.
Registered site-only in AddAuditLogHealthMetricsBridge; options bound from
AuditLog:SiteRetention. Per-tick failures swallowed. Added a test-only
PurgeIntervalOverride (mirrors SiteCallAuditOptions) so the ms-cadence tick test
can bypass the 1-min production clamp. Doc updated. (PLAN-04 Task 3, S1/U2)