feat(esg): per-system TimeoutSeconds column on ExternalSystemDefinition (spec Component-ExternalSystemGateway Timeout) — entity + EF + migration

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Entities/ExternalSystemDefinitionTests.cs

@@ -0,0 +1,19 @@
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.ExternalSystems;
+
+namespace ZB.MOM.WW.ScadaBridge.Commons.Tests.Entities;
+
+/// <summary>
+/// Locks the <see cref="ExternalSystemDefinition"/> construction invariants — in particular
+/// that the per-system <see cref="ExternalSystemDefinition.TimeoutSeconds"/> defaults to 0,
+/// meaning "unset — use the gateway's configured DefaultHttpTimeout".
+/// </summary>
+public class ExternalSystemDefinitionTests
+{
+    [Fact]
+    public void ExternalSystemDefinition_TimeoutSeconds_DefaultsToZeroMeaningUseGatewayDefault()
+    {
+        var system = new ExternalSystemDefinition("sys", "http://x", "none");
+
+        Assert.Equal(0, system.TimeoutSeconds);
+    }
+}