feat(audit-log): daily site SQLite retention purge job (closes unbounded site DB growth)

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)

src/ZB.MOM.WW.ScadaBridge.AuditLog/ServiceCollectionExtensions.cs

@@ -101,6 +101,11 @@ public static class ServiceCollectionExtensions
             .Bind(config.GetSection(SiteWriterSectionName));
         services.AddOptions<SiteAuditTelemetryOptions>()
             .Bind(config.GetSection(SiteTelemetrySectionName));
+        // Retention purge options (the SiteAuditRetentionService hosted job is
+        // registered site-only in AddAuditLogHealthMetricsBridge). Binding here is
+        // inert on central — no hosted service consumes it there.
+        services.AddOptions<SiteAuditRetentionOptions>()
+            .Bind(config.GetSection(SiteAuditRetentionOptions.SectionName));
 
         // SqliteAuditWriter is a singleton with a single owned SqliteConnection
         // and a background writer Task; multiple instances would race on the
@@ -316,6 +321,12 @@ public static class ServiceCollectionExtensions
         // free of hosted-service registrations that would resolve a missing
         // ISiteHealthCollector on central.
         services.AddHostedService<SiteAuditBacklogReporter>();
+        // Site-only retention purge: runs ISiteAuditQueue.PurgeExpiredAsync on a
+        // daily timer so the ephemeral site SQLite store does not grow unbounded.
+        // Registered here (not in AddAuditLog, which also runs on central) so the
+        // hosted service only starts on site composition roots. Guarded by the same
+        // SiteAuditBacklogReporter sentinel above against double-registration.
+        services.AddHostedService<SiteAuditRetentionService>();
         return services;
     }