feat(gateway): optional .env loading for local dev (app + seed); document in e2e guide

scripts/seed_e2e.py

@@ -28,6 +28,13 @@ import json
 import os
 import sys
 
+try:  # optional: local dev reads .env; in-cluster env comes from Helm
+    from dotenv import load_dotenv
+
+    load_dotenv()
+except ImportError:
+    pass
+
 OPC_URL = os.getenv('OPC_SERVER_URL', 'opc.tcp://localhost:49320')
 
 CONNECTOR = {