Test: verify WebSocket federation between master and child

[GeiserX]

Context

CashPilot supports multi-node fleet management via WebSocket federation. A child instance connects to a master and sends heartbeats with container state.

Task

1. Write a test that creates a master and child instance (using FastAPI TestClient)
2. Verify the child can connect via WebSocket
3. Verify heartbeat data is received by the master
4. Verify the master's fleet dashboard endpoint includes the child's data

Reference

• app/main.py — WebSocket endpoints, fleet dashboard API
• app/database.py — nodes table