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

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

src/ZB.MOM.WW.ScadaBridge.Commons/Entities/ExternalSystems/ExternalSystemDefinition.cs

@@ -17,6 +17,13 @@ public class ExternalSystemDefinition
     /// <summary>Fixed delay between retry attempts.</summary>
     public TimeSpan RetryDelay { get; set; }
 
+    /// <summary>
+    /// Per-system HTTP round-trip timeout in seconds for all method calls
+    /// (spec: Component-ExternalSystemGateway "Timeout"). 0 = unset — the gateway's
+    /// configured DefaultHttpTimeout applies.
+    /// </summary>
+    public int TimeoutSeconds { get; set; }
+
     /// <summary>
     /// Initializes a new <see cref="ExternalSystemDefinition"/>.
     /// </summary>