fix(error-handling): close Theme 4 — 18 cancellation / fire-and-forget findings

Async cancellation hygiene, fire-and-forget observability, retry/shutdown
semantics, and audit-row coverage across 9 modules. Highlights:

Cancellation & lifecycle:
- AuditLog-006: SqliteAuditWriter.Dispose hops to thread pool, escaping the
  captured SyncContext that risked sync-over-async deadlock.
- AuditLog-010: SiteAuditTelemetryActor owns a private lifecycle CTS,
  threaded through drain paths instead of CancellationToken.None.
- Comm-019: CentralCommunicationActor adds lifecycle CTS for repo calls.
- Host-019: Migration StartupRetry forwards ApplicationStopping so SIGTERM
  during the bounded-retry window aborts cleanly.

Cursor / retry / counter correctness:
- AuditLog-004: SiteAuditReconciliationActor's cursor now holds at `since`
  when any row's idempotent insert is still being retried (per-EventId
  retry counter, MaxPermanentInsertAttempts=5 escape valve with LogCritical
  abandon). No more silent abandonment of permanently-failing rows.
- ConfigDB-019: Dropped the catch-and-continue on EnsureLookaheadAsync's
  SPLIT loop — by class-doc construction the catch could only mask real
  failures and let the next iteration create permanent partition holes.
- HM-017/018: HealthReportSender + CentralHealthReportLoop snapshot
  per-interval counters before sending, restore via new
  ISiteHealthCollector.AddIntervalCounters on transport failure so counts
  aren't silently lost.

Fire-and-forget / shutdown waits:
- InboundAPI-018: AuditWriteMiddleware observes faulted audit-write tasks
  via OnlyOnFaulted continuation (Warning log; response unchanged).
- SnF-024: StoreAndForwardService.StopAsync awaits in-flight retry sweep
  with a bounded SweepShutdownWaitTimeout (10s).

Leak / refactor:
- Comm-021: SiteStreamGrpcServer.SubscribeInstance wraps Subscribe in its
  own try/catch so a throw doesn't leak the relay actor or _activeStreams
  entry.
- Comm-022: VERIFIED already-closed by Comm-016's dead-code purge.
- CLI-017: BundleCommands' three subcommands delegate to ExecuteCommandAsync
  (auth-failure exit-code contract unified).

Defensive / validation:
- CLI-021: CliConfig.Load wraps file-read/JSON parse so malformed config
  prints a warning and returns defaults instead of crashing the CLI.
- Host-022: ParseLevel emits stderr one-shot warning for unrecognised
  MinimumLevel instead of silently coercing to Information.
- ESG-019: ExternalSystemClient sets HttpClient.Timeout=Infinite so the
  per-call CTS is the sole timeout source (was clipped to 100s by .NET).
- Security-020: New SecurityOptionsValidator (IValidateOptions) rejects
  empty LdapServer/LdapSearchBase with ValidateOnStart.
- DM-019: Lifecycle command timeouts now emit DisableTimedOut/EnableTimedOut/
  DeleteTimedOut audit entries (mirrors DeployFailed pattern).

Plus reconciled stale per-module Open-findings counters that had drifted
from prior sessions.

20+ new regression tests across 11 test projects; build clean; affected
suites all green. README regenerated: 75 open (was 93).

src/ScadaLink.Host/Program.cs

@@ -161,18 +161,23 @@ try
             // exponential backoff before failing fatally.
             // Host-015: only connection-class (transient) faults are retried — a
             // schema-version mismatch is permanent and must fail fast on attempt 1.
+            // Host-019: thread the host's ApplicationStopping token into both the
+            // migration call itself and the inter-attempt Task.Delay so a SIGTERM
+            // during the bounded-retry window (~2 min worst-case) tears down
+            // cleanly instead of being ignored until the loop exhausts.
             await StartupRetry.ExecuteWithRetryAsync(
                 "database-migration",
-                async () =>
+                async ct =>
                 {
                     using var scope = app.Services.CreateScope();
                     var dbContext = scope.ServiceProvider.GetRequiredService<ScadaLinkDbContext>();
-                    await MigrationHelper.ApplyOrValidateMigrationsAsync(dbContext, isDevelopment, migrationLogger);
+                    await MigrationHelper.ApplyOrValidateMigrationsAsync(dbContext, isDevelopment, migrationLogger, ct);
                 },
                 maxAttempts: 8,
                 initialDelay: TimeSpan.FromSeconds(2),
                 migrationLogger,
-                isTransient: StartupRetry.IsTransientDatabaseFault);
+                isTransient: StartupRetry.IsTransientDatabaseFault,
+                cancellationToken: app.Lifetime.ApplicationStopping);
         }
 
         // Middleware pipeline