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.ManagementService.Tests/ManagementActorTests.cs

@@ -896,11 +896,15 @@ public class ManagementActorTests : TestKit, IDisposable
     [Fact]
     public void QueryDeployments_UnfilteredForSiteScopedUser_DropsOutOfScopeRecords()
     {
-        // Records for instances 1 (site 1, in scope) and 2 (site 2, out of scope).
-        _templateRepo.GetInstanceByIdAsync(1, Arg.Any<CancellationToken>())
-            .Returns(new Instance("Pump1") { Id = 1, SiteId = 1 });
-        _templateRepo.GetInstanceByIdAsync(2, Arg.Any<CancellationToken>())
-            .Returns(new Instance("Pump2") { Id = 2, SiteId = 2 });
+        // ManagementService-023: the unfiltered branch now bulk-loads instances
+        // once via GetAllInstancesAsync (instead of N+1 GetInstanceByIdAsync per
+        // distinct InstanceId). Mock the bulk path accordingly.
+        _templateRepo.GetAllInstancesAsync(Arg.Any<CancellationToken>())
+            .Returns(new List<Instance>
+            {
+                new("Pump1") { Id = 1, SiteId = 1 },
+                new("Pump2") { Id = 2, SiteId = 2 },
+            });
         var deployRepo = Substitute.For<IDeploymentManagerRepository>();
         deployRepo.GetAllDeploymentRecordsAsync(Arg.Any<CancellationToken>())
             .Returns(new List<Commons.Entities.Deployment.DeploymentRecord>
@@ -917,6 +921,43 @@ public class ManagementActorTests : TestKit, IDisposable
         var response = ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
         Assert.Contains("deploy-1", response.JsonData);
         Assert.DoesNotContain("deploy-2", response.JsonData);
+        // The per-instance lookup must NOT have been used for the unfiltered
+        // branch — that was the N+1 the bulk load replaced.
+        _templateRepo.DidNotReceiveWithAnyArgs().GetInstanceByIdAsync(default);
+    }
+
+    [Fact]
+    public void QueryDeployments_UnfilteredForSiteScopedUser_UsesBulkInstanceLoad_NotPerRecordLookup()
+    {
+        // ManagementService-023 regression pin: the unfiltered branch must issue
+        // GetAllInstancesAsync ONCE and never call GetInstanceByIdAsync, regardless
+        // of how many DeploymentRecords reference distinct InstanceIds. Before the
+        // fix, three distinct instance ids would have produced three per-instance
+        // lookups (textbook N+1).
+        _templateRepo.GetAllInstancesAsync(Arg.Any<CancellationToken>())
+            .Returns(new List<Instance>
+            {
+                new("Pump1") { Id = 1, SiteId = 1 },
+                new("Pump2") { Id = 2, SiteId = 2 },
+                new("Pump3") { Id = 3, SiteId = 1 },
+            });
+        var deployRepo = Substitute.For<IDeploymentManagerRepository>();
+        deployRepo.GetAllDeploymentRecordsAsync(Arg.Any<CancellationToken>())
+            .Returns(new List<Commons.Entities.Deployment.DeploymentRecord>
+            {
+                DeploymentRecordFor(1), DeploymentRecordFor(2), DeploymentRecordFor(3),
+                DeploymentRecordFor(1), DeploymentRecordFor(3) // duplicates: still no extra lookups
+            });
+        _services.AddScoped(_ => deployRepo);
+
+        var actor = CreateActor();
+        var envelope = ScopedEnvelope(new QueryDeploymentsCommand(), new[] { "1" }, "Deployment");
+
+        actor.Tell(envelope);
+
+        ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        _templateRepo.Received(1).GetAllInstancesAsync(Arg.Any<CancellationToken>());
+        _templateRepo.DidNotReceiveWithAnyArgs().GetInstanceByIdAsync(default);
     }
 
     [Fact]