Add AI engine load balancer authentication

app/common/src/main/java/stirling/software/common/model/ApplicationProperties.java

@@ -283,6 +283,7 @@ public class ApplicationProperties {
 
         private Valkey valkey = new Valkey();
         private Node node = new Node();
+        private Engine engine = new Engine();
 
         private transient String cachedNodeId;
 
@@ -354,6 +355,16 @@ public class ApplicationProperties {
             /** Heartbeat publish interval for the instance registry, in milliseconds. */
             private long heartbeatIntervalMs = 5000;
         }
+
+        @Data
+        public static class Engine {
+            /**
+             * Shared secret for the {@code X-Engine-Auth} header on Java -> Python engine calls.
+             * Must match {@code STIRLING_ENGINE_SHARED_SECRET} on the engine pod. Blank = dev mode
+             * (no auth) - never leave blank in production.
+             */
+            private String sharedSecret = "";
+        }
     }
 
     /**