Run PostgreSQL in `platform-gateway-ha-test.sh` for a full integration test

[michalrus]

Specifically after 243271b, we don’t have a real IceBreakers registration test.

There's only a fake TestGateway:

blockfrost-platform/crates/integration_tests/src/gateway/mod.rs

Lines 61 to 68 in 2712878

	/// A self-contained gateway with mock `/register` for E2E tests
	pub struct TestGateway {
	pub addr: SocketAddr,
	pub lb: LoadBalancerState,
	pub rate_limiter: RegisterRateLimiter,
	shutdown_tx: Option<oneshot::Sender<()>>,
	server_handle: JoinHandle<()>,
	}

[michalrus]

(I'm dumping my personal notes to the issue tracker for the future.)