fix(options): validate Host Node/Database/Logging options at startup — empty NodeName fails fast instead of NULLing SourceNode (plan R2-08 T7, arch-review 08r2 NF4)

src/ZB.MOM.WW.ScadaBridge.Host/DatabaseOptionsValidator.cs

@@ -0,0 +1,28 @@
+using ZB.MOM.WW.Configuration;
+
+namespace ZB.MOM.WW.ScadaBridge.Host;
+
+/// <summary>
+/// Validates <see cref="DatabaseOptions"/> at host startup (arch-review 08
+/// round 2 NF4). All three connection settings are nullable and role-dependent
+/// (central needs the SQL Server strings, a site needs only the SQLite path),
+/// so a <c>null</c> value is valid. A present-but-whitespace value is not — it
+/// signals a mis-set config key that would fail opaquely at first DB use.
+/// </summary>
+public sealed class DatabaseOptionsValidator : OptionsValidatorBase<DatabaseOptions>
+{
+    /// <inheritdoc />
+    protected override void Validate(ValidationBuilder builder, DatabaseOptions options)
+    {
+        RequireNotWhitespace(builder, options.ConfigurationDb, nameof(DatabaseOptions.ConfigurationDb));
+        RequireNotWhitespace(builder, options.MachineDataDb, nameof(DatabaseOptions.MachineDataDb));
+        RequireNotWhitespace(builder, options.SiteDbPath, nameof(DatabaseOptions.SiteDbPath));
+    }
+
+    // null is valid (role-dependent); reject only a present-but-blank value.
+    private static void RequireNotWhitespace(ValidationBuilder builder, string? value, string field)
+    {
+        builder.RequireThat(value is null || !string.IsNullOrWhiteSpace(value),
+            $"ScadaBridge:Database:{field} is set but blank — either remove it or give it a real value.");
+    }
+}