feat(config): make BASE_URL a required field and update related documentation

backend/.env.example

@@ -7,6 +7,7 @@ REQUEST_LOG_INCLUDE_HEALTH=false
 DATABASE_URL=postgresql+psycopg://postgres:postgres@localhost:5432/mission_control
 # For remote access, set this to your UI origin (e.g. http://<server-ip>:3000 or https://mc.example.com).
 CORS_ORIGINS=http://localhost:3000
+# REQUIRED for gateway provisioning/agent heartbeats. Must be reachable by gateway runtime.
 BASE_URL=
 # Security response headers (blank values disable each header).
 SECURITY_HEADER_X_CONTENT_TYPE_OPTIONS=