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/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;
 }