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).

code-reviews/AuditLog/findings.md

@@ -8,7 +8,7 @@
 | Last reviewed | 2026-05-28 |
 | Reviewer | claude-agent |
 | Commit reviewed | `1eb6e97` |
-| Open findings | 3 |
+| Open findings | 2 |
 
 ## Summary
 
@@ -263,7 +263,7 @@ tests already exercise the success path).
 |--|--|
 | Severity | Medium |
 | Category | Performance & resource management |
-| Status | Open |
+| Status | Resolved |
 | Location | `src/ScadaLink.AuditLog/Site/SqliteAuditWriter.cs:597-657` |
 
 **Description**
@@ -293,9 +293,20 @@ lazily on a dedicated background tick so the reporter reads a pre-computed snaps
 without acquiring the write lock. Option (a) also unblocks `ReadPendingAsync` /
 `ReadPendingSinceAsync` from competing with the writer.
 
-**Resolution**
+**Resolution (2026-05-28):**
 
-_Unresolved._
+Took option (a). `SqliteAuditWriter` now opens a second `SqliteConnection`
+(`_readConnection`) on the same file in the ctor, after `InitializeSchema`
+sets `PRAGMA journal_mode = WAL` on the writer connection — WAL lets a
+second connection read concurrently with the active writer without taking
+`_writeLock`. The read connection is guarded by its own `_readLock` (since
+`SqliteConnection` itself is not thread-safe across callers) and used by
+`GetBacklogStatsAsync`, `ReadPendingAsync`, `ReadPendingSinceAsync`, and
+`ReadForwardedAsync`. `DisposeAsync` disposes it after the writer drains.
+Regression test `SqliteAuditWriterBacklogStatsTests.GetBacklogStatsAsync_DoesNotBlockOnConcurrentWriteLoad`
+saturates the writer with a 2 000-row burst and asserts the probe returns
+in under 1 s — would fail against the pre-fix code (the probe queued
+behind every batch INSERT under `_writeLock`).
 
 ### AuditLog-006 — `SqliteAuditWriter.Dispose()` does sync-over-async and may deadlock