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/CLI/findings.md

@@ -8,7 +8,7 @@
 | Last reviewed | 2026-05-28 |
 | Reviewer | claude-agent |
 | Commit reviewed | `1eb6e97` |
-| Open findings | 4 |
+| Open findings | 3 |
 
 ## Summary
 
@@ -888,9 +888,11 @@ and pass after.
 |--|--|
 | Severity | Medium |
 | Category | Performance & resource management |
-| Status | Open |
+| Status | Resolved |
 | Location | `src/ScadaLink.CLI/Commands/BundleCommands.cs:117-124`, `src/ScadaLink.CLI/ManagementHttpClient.cs:47-92` |
 
+**Resolution (2026-05-28):** Replaced the `Convert.FromBase64String(...)` + `File.WriteAllBytes(...)` pair with a new `StreamBase64ToFile(base64, output)` helper that slices the base64 string into 4-char-aligned chunks (1 MB by default) and decodes each chunk straight into a `FileStream` via `Convert.TryFromBase64Chars`. The intermediate `byte[]` and the synchronous full-bundle write are gone — peak working set drops from ~base64-string + ~byte[] + ~envelope-string to ~base64-string + small-chunk-buffer + ~envelope-string. The response body is still buffered into the management envelope string (the `POST /management` wire format does not currently support streaming responses — this finding is bounded by that limit per the recommendation's stop-gap), so a streaming `POST /api/bundle/export` REST endpoint remains a follow-up. Regression tests `BundleCommandsStreamingTests` (6 tests) cover small payload round-trip, multi-chunk boundaries, empty input, invalid base64 → `FormatException`, and argument validation.
+
 **Description**
 
 `Component-Transport.md:271` ceilings the raw bundle at 100 MB and notes the