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

ingestor/app.py

@@ -7,6 +7,13 @@ from time import time
 
 from prometheus_client import start_http_server
 
+try:  # optional: local dev reads .env; in-cluster env comes from Helm (no .env file)
+    from dotenv import load_dotenv
+
+    load_dotenv()
+except ImportError:
+    pass
+
 import ingestor.metrics as metrics
 from ingestor.config import GatewayConfig
 from ingestor.gateway_runtime import GatewayRuntime