feat(auditlog): make SiteAuditBacklogReporter cadence configurable (deferred #21)

Consolidates the reporter's poll cadence onto SqliteAuditWriterOptions instead of
the hard-coded 30 s constant, closing deferred-work register item #21 (the config-
shape cleanup its own XML doc flagged as a follow-up).

- SqliteAuditWriterOptions.BacklogPollIntervalSeconds (default 30).
- Reporter reads it via IOptions; precedence explicit-override (tests) > configured
  > 30 s default; a non-positive value falls back to the default. Corrected the
  stale "hard-code / tunable in a follow-up" class-doc.
- Cadence tests + register updated (row 21 -> Resolved).

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

src/ZB.MOM.WW.ScadaBridge.AuditLog/Site/SiteAuditBacklogReporter.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
 using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Services;
 using ZB.MOM.WW.ScadaBridge.HealthMonitoring;
 
@@ -25,9 +26,8 @@ namespace ZB.MOM.WW.ScadaBridge.AuditLog.Site;
 /// <b>Cadence.</b> 30 s by default — coarse enough to amortise the SQL probe
 /// across many reports, fine enough that the central dashboard never lags by
 /// more than one health-report interval. Tunable via
-/// <see cref="ZB.MOM.WW.ScadaBridge.AuditLog.Site.SqliteAuditWriterOptions"/> in a follow-up
-/// if ops needs a different cadence; for now we hard-code the value because the
-/// brief calls it out explicitly.
+/// <see cref="ZB.MOM.WW.ScadaBridge.AuditLog.Site.SqliteAuditWriterOptions.BacklogPollIntervalSeconds"/>
+/// (a non-positive value falls back to the 30 s default).
 /// </para>
 /// <para>
 /// <b>Failure containment.</b> The probe call is wrapped in a try/catch so a
@@ -56,17 +56,42 @@ public sealed class SiteAuditBacklogReporter : IHostedService, IDisposable
     /// <param name="queue">The site audit queue used to probe the backlog count.</param>
     /// <param name="collector">The site health collector that receives the backlog snapshot.</param>
     /// <param name="logger">Logger instance.</param>
-    /// <param name="refreshInterval">Poll interval override; defaults to <see cref="DefaultRefreshInterval"/> (30 s).</param>
+    /// <param name="refreshInterval">
+    /// Explicit poll-interval override (used by tests). When null, the cadence comes from
+    /// <see cref="SqliteAuditWriterOptions.BacklogPollIntervalSeconds"/>, falling back to
+    /// <see cref="DefaultRefreshInterval"/> (30 s) when that is unset or non-positive.
+    /// </param>
+    /// <param name="options">
+    /// SQLite audit-writer options supplying the configurable poll cadence
+    /// (<see cref="SqliteAuditWriterOptions.BacklogPollIntervalSeconds"/>). Optional so
+    /// direct construction in tests need not build the options graph.
+    /// </param>
     public SiteAuditBacklogReporter(
         ISiteAuditQueue queue,
         ISiteHealthCollector collector,
         ILogger<SiteAuditBacklogReporter> logger,
-        TimeSpan? refreshInterval = null)
+        TimeSpan? refreshInterval = null,
+        IOptions<SqliteAuditWriterOptions>? options = null)
     {
         _queue = queue ?? throw new ArgumentNullException(nameof(queue));
         _collector = collector ?? throw new ArgumentNullException(nameof(collector));
         _logger = logger ?? throw new ArgumentNullException(nameof(logger));
-        _refreshInterval = refreshInterval ?? DefaultRefreshInterval;
+        // Precedence: explicit override (tests) → configured cadence → 30 s default.
+        _refreshInterval = refreshInterval ?? ResolveInterval(options) ?? DefaultRefreshInterval;
+    }
+
+    /// <summary>The resolved poll cadence (exposed for tests).</summary>
+    internal TimeSpan RefreshInterval => _refreshInterval;
+
+    /// <summary>
+    /// Maps <see cref="SqliteAuditWriterOptions.BacklogPollIntervalSeconds"/> to a
+    /// <see cref="TimeSpan"/>, or null when unconfigured / non-positive (caller then
+    /// falls back to <see cref="DefaultRefreshInterval"/>).
+    /// </summary>
+    private static TimeSpan? ResolveInterval(IOptions<SqliteAuditWriterOptions>? options)
+    {
+        var seconds = options?.Value.BacklogPollIntervalSeconds ?? 0;
+        return seconds > 0 ? TimeSpan.FromSeconds(seconds) : null;
     }
 
     /// <summary>Starts the background polling loop, running an immediate first probe before entering the timed cycle.</summary>

src/ZB.MOM.WW.ScadaBridge.AuditLog/Site/SqliteAuditWriterOptions.cs

@@ -24,4 +24,13 @@ public sealed class SqliteAuditWriterOptions
 
     /// <summary>Soft flush interval the writer enforces when fewer than BatchSize events are queued.</summary>
     public int FlushIntervalMs { get; set; } = 50;
+
+    /// <summary>
+    /// Poll cadence, in seconds, for the <c>SiteAuditBacklogReporter</c> hosted service
+    /// that probes the SQLite audit backlog and pushes the snapshot onto the site health
+    /// report. Half a typical 60 s health-report interval keeps the snapshot fresh without
+    /// spinning the SQL probe more often than necessary. A non-positive value falls back to
+    /// the reporter's 30 s default.
+    /// </summary>
+    public int BacklogPollIntervalSeconds { get; set; } = 30;
 }