feat(runtime): per-script execution timeout overriding the global default (#9)

Spec promised a per-script timeout but only the global ScriptExecutionTimeoutSeconds
existed. Add nullable TemplateScript.ExecutionTimeoutSeconds threaded through EF +
flattening (ResolvedScript) to ScriptExecutionActor/AlarmExecutionActor, which use
perScript ?? global for the execution CTS. Includes the EF migration for the new column.

src/ZB.MOM.WW.ScadaBridge.Commons/Entities/Templates/TemplateScript.cs

@@ -52,6 +52,15 @@ public class TemplateScript
     /// </summary>
     public TimeSpan? MinTimeBetweenRuns { get; set; }
 
+    /// <summary>
+    /// Per-script execution timeout in seconds, or null to use the site's global
+    /// default (<c>SiteRuntimeOptions.ScriptExecutionTimeoutSeconds</c>). A
+    /// non-positive value (≤ 0) is treated the same as null — i.e. fall back to
+    /// the global default — by the Site Runtime. Seconds (not a TimeSpan) to keep
+    /// the unit consistent with the global option it overrides.
+    /// </summary>
+    public int? ExecutionTimeoutSeconds { get; set; }
+
     /// <summary>
     /// True when this row was copied from the base template and has not been
     /// overridden on the derived template. Changes to the base flow downward