Deploy the staging gateway pointed at the local testnet Bee

[crtahlin]

Part of #240. Depends on #SUB2.

Deploy the staging gateway container on the VM, pointed at the local testnet Bee.

Scope

• Run the gateway via Docker Compose on the VM; BEE_API_URL → the local testnet Bee.
• Staging env: GATEWAY_ENVIRONMENT=staging, GNOSIS_CHAIN_ID=11155111, testnet GNOSIS_RPC_URL, Sepolia PostageStamp/BZZ addresses default per chain id. Flow B left off here (enabled in #SUB8).
• Wire metrics/Grafana Alloy as today.

Acceptance criteria

• GET /health returns ok and shows the bee_node section with the testnet node's peer/availability info.
• GET /api/v1/debug/bee/topology works (debug allow-list configured) and reflects the testnet node.
• Gateway restarts cleanly (compose restart: unless-stopped or equivalent).
• No mainnet Bee or mainnet chain references in the staging env.

[crtahlin]

Partly done, with the premise changed.

Done: the staging gateway points at a Bee node on the same host, reached over the compose network as http://bee-dev:1633. The Bee API is not published — it has no authentication, so it is reachable only from within the compose network.

Not as planned: that node is mainnet, not testnet (see #242).

Leaving open until #242 is decided, since what "local testnet Bee" should mean depends on that answer.