fix(auditlog): page the site audit purge, bound incremental_vacuum per page, fail fast instead of parking writers on a full queue (review 2026-10-21 06/MEM-1)

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

@@ -29,8 +29,11 @@ public sealed class SqliteAuditWriterOptions
 
     /// <summary>
     /// Capacity of the bounded write queue. Set high enough that ordinary
-    /// script bursts never fill it; <see cref="System.Threading.Channels.BoundedChannelFullMode.Wait"/>
-    /// applies when the writer falls behind.
+    /// script bursts never fill it. When the writer falls behind and the queue is
+    /// full, <c>WriteAsync</c> fails fast with <see cref="SqliteAuditWriterSaturatedException"/>
+    /// instead of parking an unbounded number of waiters (review 2026-10-21 06/MEM-1);
+    /// <see cref="FallbackAuditWriter"/> turns that into its documented best-effort
+    /// drop-oldest ring + <c>SiteAuditWriteFailures</c> counter bump.
     /// </summary>
     public int ChannelCapacity { get; set; } = 4096;
 
@@ -40,6 +43,23 @@ 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>
+    /// Maximum rows the retention purge deletes in ONE transaction. The purge loops page by
+    /// page and releases the write lock between pages, so the batched hot-path writer
+    /// interleaves with a large purge instead of stalling behind one multi-minute DELETE
+    /// (review 2026-10-21 06/MEM-1 — the same hazard class register row 34 fixed for
+    /// <c>site_events</c>).
+    /// </summary>
+    public int PurgeBatchSize { get; set; } = 5000;
+
+    /// <summary>
+    /// Free pages reclaimed by the bounded <c>PRAGMA incremental_vacuum(N)</c> run after each
+    /// purge page. Bounds the time the vacuum holds the write lock; pages not reclaimed in
+    /// one step stay on the freelist and are reused by later inserts or reclaimed by the
+    /// next purge.
+    /// </summary>
+    public int PurgeVacuumPagesPerBatch { get; set; } = 1000;
+
     /// <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