GatewayRuntime builds and owns the F4 object graph (CertProvider/pool ->
WriteEngine/NatsWriteConsumer -> ActiveReadEngine -> optional GrpcServer),
reusing the existing ingestor's logger/notification_handler/metrics_controller
and DataManager (CertProvider shares its Mongo connection instead of opening
a second one). app.py starts it after prepare_ingestor, sweeps idle OPC
connections each loop iteration, and stops it on shutdown. A NATS connect
failure at startup is logged/notified but never crashes the app -- the read
path keeps working and idle_sweep() retries the consumer start.
ingestor.py needed no changes: data_manager is already reachable via
ingestor.ingestor_manager.data_manager.
Also documents the new MinIO/Postgres env vars this wiring reads (via
sientia_do's build_postgres_config for Postgres) in .env.example, and
updates test_app.py's fixture to mock GatewayConfig/GatewayRuntime so the
existing app tests keep exercising only what they mocked before.
