fix(audit): maintenance-login roll-forward/backfill, archive orphan cleanup, head-export health (ScadaBridge#113 #114 #115)

#113: AuditLogPartitionMaintenance (monthly SPLIT RANGE) and the
backfill-source-node endpoint now use the AuditLog:MaintenanceConnectionString
login when set (unset = unchanged). Migration AuditMaintenanceGrants gives
scadabridge_audit_purger ALTER ANY DATASPACE and column-level UPDATE (SourceNode).
#114: archive cleanup deletes the writer's own *.tmp files and manifest-less
data files older than 24 h inside {Directory}/{YYYY} (DeletedOrphan, dry-run aware).
#115: head-export failures bump scadabridge.audit.chain_head_export.failures and
Degrade the /health/ready entry audit-chain-head-export (failing or stale > 2 intervals).

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/ZB.MOM.WW.ScadaBridge.Host/Health/AuditChainHeadExportHealthCheck.cs

@@ -0,0 +1,63 @@
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+using ZB.MOM.WW.ScadaBridge.AuditLog.Central.HeadExport;
+
+namespace ZB.MOM.WW.ScadaBridge.Host.Health;
+
+/// <summary>
+/// /health/ready entry (ScadaBridge#115): Degraded while the off-box AuditLog chain-head export
+/// (<c>AuditLog:HashChain:HeadExport</c>) is failing, or has had no success for more than two
+/// intervals. Degraded maps to HTTP 200, so it shows on the Overview dashboard without taking
+/// the node out of rotation. Healthy when the export is disabled.
+/// </summary>
+public sealed class AuditChainHeadExportHealthCheck : IHealthCheck
+{
+    private readonly AuditChainHeadExportStatus _status;
+    private readonly TimeProvider _time;
+
+    /// <summary>Initializes the check.</summary>
+    /// <param name="status">The export status singleton.</param>
+    /// <param name="time">Clock (defaults to the system clock).</param>
+    public AuditChainHeadExportHealthCheck(AuditChainHeadExportStatus status, TimeProvider? time = null)
+    {
+        _status = status ?? throw new ArgumentNullException(nameof(status));
+        _time = time ?? TimeProvider.System;
+    }
+
+    /// <inheritdoc />
+    public Task<HealthCheckResult> CheckHealthAsync(
+        HealthCheckContext context, CancellationToken cancellationToken = default)
+    {
+        if (!_status.Enabled)
+        {
+            return Task.FromResult(HealthCheckResult.Healthy("Chain-head export disabled."));
+        }
+
+        var now = _time.GetUtcNow().UtcDateTime;
+        var data = new Dictionary<string, object>(StringComparer.Ordinal)
+        {
+            ["totalFailures"] = _status.TotalFailures,
+            ["consecutiveFailures"] = _status.ConsecutiveFailures,
+            ["intervalMinutes"] = _status.IntervalMinutes,
+        };
+        if (_status.LastSuccessUtc is { } ok)
+        {
+            data["lastSuccessUtc"] = ok;
+        }
+
+        if (_status.ConsecutiveFailures > 0)
+        {
+            return Task.FromResult(HealthCheckResult.Degraded(
+                $"Chain-head export failing ({_status.ConsecutiveFailures} consecutive): {_status.LastError}", data: data));
+        }
+
+        var limit = TimeSpan.FromMinutes(2L * _status.IntervalMinutes);
+        var reference = _status.LastSuccessUtc ?? _status.StartedUtc;
+        if (reference is { } r && now - r > limit)
+        {
+            return Task.FromResult(HealthCheckResult.Degraded(
+                $"No successful chain-head export for more than 2 intervals (last {(_status.LastSuccessUtc is null ? "never" : _status.LastSuccessUtc.Value.ToString("O"))}).", data: data));
+        }
+
+        return Task.FromResult(HealthCheckResult.Healthy("Chain-head export OK.", data));
+    }
+}

src/ZB.MOM.WW.ScadaBridge.Host/Program.cs

@@ -579,6 +579,12 @@ try
                 "bundle-signing-policy",
                 failureStatus: null,
                 tags: new[] { ZbHealthTags.Ready })
+            // ScadaBridge#115: Degraded (HTTP 200) while the off-box chain-head export is failing
+            // or stale; healthy when the export is disabled.
+            .AddTypeActivatedCheck<AuditChainHeadExportHealthCheck>(
+                "audit-chain-head-export",
+                failureStatus: null,
+                tags: new[] { ZbHealthTags.Ready })
             // Oldest-Up-member check (review 01 [High]), unscoped: every central member competes for
             // the one active slot. Since ZB.MOM.WW.Health 0.3.0 this IS the shared check — it selects
             // by age, so the host-local OldestNodeActiveHealthCheck that existed only to avoid the old