feat(historian): backend registry, ServerHistorian:Backend, Host wiring via provider

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Server/ZB.MOM.WW.OtOpcUa.Host/Configuration/ServerHistorianOptionsValidator.cs

@@ -57,12 +57,22 @@ public sealed class ServerHistorianOptionsValidator : OptionsValidatorBase<Serve
     /// <inheritdoc />
     protected override void Validate(ValidationBuilder builder, ServerHistorianOptions options)
     {
+        // Unknown backend always fails, with no I/O and regardless of the consumer gate.
+        builder.RequireThat(
+            HistorianBackendRegistry.IsKnown(options.Backend),
+            $"ServerHistorian:Backend '{options.Backend}' is not a known historian backend. "
+            + $"Allowed values: {string.Join(", ", HistorianBackendRegistry.KnownBackends)}.");
+
         var alarmHistorianEnabled = _configuration.GetValue<bool>("AlarmHistorian:Enabled");
         var consumed = options.Enabled || alarmHistorianEnabled;
 
         // A section nobody consumes may legitimately be empty (the docker-dev default) — no checks.
         if (!consumed) return;
 
+        // Gateway connection checks apply only to the Gateway backend; other backends validate their own
+        // options (added with the backend) here, under the same consumer gate.
+        if (!options.IsGatewayBackend) return;
+
         var endpointValid = Uri.TryCreate(options.Endpoint, UriKind.Absolute, out var uri)
             && (uri.Scheme == Uri.UriSchemeHttp || uri.Scheme == Uri.UriSchemeHttps);