perf: close Theme 6 — 11 allocation / N+1 / lock-contention findings

Well-localised perf fixes across 8 modules.

Lock decoupling / SQL streaming:
- AuditLog-005: SqliteAuditWriter gains dedicated read-only _readConnection
  (+ _readLock) backed by WAL journal mode. GetBacklogStatsAsync,
  ReadPendingAsync, ReadPendingSinceAsync, ReadForwardedAsync no longer
  contend with the hot-path INSERT lock — backlog probes on a 30s timer
  can't stall the writer under multi-hundred-K Pending backlog.
- SEL-022: dropped Cache=Shared from SiteEventLogger's default connection
  string (single-connection logger; mode was dormant config).

Memory / streaming:
- CLI-019: bundle export streams base64 in 1 MB-aligned chunks via
  Convert.TryFromBase64Chars straight into the FileStream — no more
  full-bundle byte[] allocation.
- CentralUI-031: TransportImport now stages the upload to a per-session
  temp file under Path.GetTempPath() (replaces in-memory byte[] field);
  page implements IDisposable to delete the temp file on reset / new
  upload / dispose. Per-circuit working set drops from ~100 MB to ~80 KB.

N+1 hoisting:
- Transport-008: added ITemplateEngineRepository.GetTemplatesWithChildrenAsync
  bulk method; BundleImporter.PreviewAsync calls it once instead of per-
  template-name. Single query with .Include(...).AsSplitQuery().
- DM-023: BuildDeployArtifactsCommandAsync's per-site loop now references
  a pre-fetched GlobalArtifactSnapshot (shared scripts, external systems,
  DB connections, notification lists, SMTP) instead of re-querying per site.
- MgmtSvc-023: HandleQueryDeployments unfiltered branch uses one
  GetAllInstancesAsync bulk load + Dictionary<int,int?> lookup (was a
  GetInstanceByIdAsync per record).

Small allocations / per-tick rebuilds:
- InboundAPI-019: AuditWriteMiddleware gates EnableBuffering() on
  RequestHasBody() so GET/HEAD/DELETE/TRACE/OPTIONS and Content-Length:0
  requests skip the FileBufferingReadStream allocation.
- NotifOutbox-006: ResolveAdapters dictionary now cached on
  _adaptersCache (built lazily on first sweep) + actor-lifetime
  _adaptersScope; ResolveAdapters no longer rebuilds per dispatch tick.

Verify-only:
- Comm-017: Confirmed _inProgressDeployments was deleted by Comm-016 in
  commit ac96b83 — marked Resolved with that attribution. No code change.

Doc-correction:
- NS-022: Updated MailKitSmtpClientWrapper XML doc to spell out single-
  connection / per-delivery-factory contract (option (b) — transient
  client per Send — rejected because it re-handshakes TLS per email).

10+ new regression tests across 8 test projects. Build clean; affected
suites all green. README regenerated: 54 open (was 65).

tests/ScadaLink.Transport.IntegrationTests/Import/BundleImporterPreviewTests.cs

@@ -338,4 +338,52 @@ public sealed class BundleImporterPreviewTests : IDisposable
                 i.Kind == ConflictKind.Blocker && i.Name == name);
         }
     }
+
+    [Fact]
+    public async Task PreviewAsync_multiple_templates_with_children_diffs_each_correctly()
+    {
+        // Transport-008 regression: PreviewAsync previously fetched each matching
+        // template's children via a per-name GetTemplateWithChildrenAsync call
+        // (N+1). The bulk variant returns every match in a single query — this
+        // test seeds three templates with distinct child collections and asserts
+        // the preview hydrates each one so the per-child diff sees the right
+        // attribute / alarm / script counts (i.e. the bulk fetch did not lose
+        // any child rows compared to the per-name fetch).
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var ctx = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
+            var pump = new Template("Pump") { Description = "p1" };
+            pump.Attributes.Add(new TemplateAttribute("Flow"));
+            pump.Scripts.Add(new TemplateScript("init", "return 1;"));
+            var valve = new Template("Valve") { Description = "v1" };
+            valve.Alarms.Add(new TemplateAlarm("HighPressure"));
+            var tank = new Template("Tank") { Description = "t1" };
+            tank.Attributes.Add(new TemplateAttribute("Level"));
+            tank.Attributes.Add(new TemplateAttribute("Temperature"));
+            ctx.Templates.AddRange(pump, valve, tank);
+            await ctx.SaveChangesAsync();
+        }
+
+        var bundleStream = await ExportTemplatesAsync();
+        var bytes = await StreamToBytes(bundleStream);
+
+        ImportPreview preview;
+        await using (var scope = _provider.CreateAsyncScope())
+        {
+            var importer = scope.ServiceProvider.GetRequiredService<IBundleImporter>();
+            var session = await importer.LoadAsync(new MemoryStream(bytes), passphrase: null);
+            preview = await importer.PreviewAsync(session.SessionId);
+        }
+
+        // Each template should be diff-classified (Identical, since the bundle
+        // is the literal projection of the target). Critically, the diff must
+        // succeed for ALL three — a bulk-fetch bug that silently drops rows
+        // would surface here as a missing item or a wrong (New) classification.
+        var pumpItem = Assert.Single(preview.Items, i => i.EntityType == "Template" && i.Name == "Pump");
+        var valveItem = Assert.Single(preview.Items, i => i.EntityType == "Template" && i.Name == "Valve");
+        var tankItem = Assert.Single(preview.Items, i => i.EntityType == "Template" && i.Name == "Tank");
+        Assert.Equal(ConflictKind.Identical, pumpItem.Kind);
+        Assert.Equal(ConflictKind.Identical, valveItem.Kind);
+        Assert.Equal(ConflictKind.Identical, tankItem.Kind);
+    }
 }