feat(health): wire ISiteEventLogger.FailedWriteCount into SiteHealthReport (#30, M2.16)

Add SiteHealthReport.SiteEventLogWriteFailures (trailing optional long = 0,
additive-only), ISiteHealthCollector.SetSiteEventLogWriteFailures (default
no-op so existing fakes compile), and SiteEventLogFailureCountReporter
(hosted service in HealthMonitoring, Func<long> delegate to avoid the
HealthMonitoring → StoreAndForward → SiteEventLogging cycle).

Registration helper AddSiteEventLogHealthMetricsBridge added to
HealthMonitoring.ServiceCollectionExtensions; wired in
SiteServiceRegistration after AddSiteEventLogging.

Tests: SiteEventLogWriteFailuresMetricTests (4 collector tests) +
SiteEventLogFailureCountReporterTests (2 poller tests) in
HealthMonitoring.Tests. 79/79 HealthMonitoring.Tests green,
59/59 SiteEventLogging.Tests green, 0 warnings.

src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Health/SiteHealthReport.cs

@@ -40,7 +40,14 @@ public record SiteHealthReport(
     // hosted service every 30 s. Defaults to null so existing producers /
     // tests that don't refresh the snapshot stay valid; the central health
     // surface treats null as "no data yet" rather than a zeroed queue.
-    SiteAuditBacklogSnapshot? SiteAuditBacklog = null);
+    SiteAuditBacklogSnapshot? SiteAuditBacklog = null,
+    // Site Event Logging (#12) M2.16 (#30): cumulative count of event-log write
+    // failures (SQLite error, disk full, bounded-queue overflow drop) since the
+    // logger was created. Populated by the site-side SiteEventLogFailureCountReporter
+    // hosted service. Point-in-time (not reset on collect) — mirrors the
+    // SiteAuditBacklog pattern. Defaults to 0 so existing producers / tests that
+    // don't wire the poller stay valid.
+    long SiteEventLogWriteFailures = 0);
 
 /// <summary>
 /// Broadcast wrapper used between central nodes to keep per-node