Define the events/v1 GameFinished contract and add RabbitMQ to the stack

[TexablePlum]

Decision from the 2026-07 architecture review: introduce RabbitMQ as the domain event bus.
Deliberately out of scope: STOMP broker relay (separate scaling milestone), engine calls,
matchmaking pairing.

Scope

• checkers-contracts/events/v1/: versioned envelope schema (type, version, eventId as the
  idempotency key, occurredAt, payload) + GameFinished payload schema + README documenting the
  fanout exchange topology, per-consumer queues with DLQs, at-least-once semantics and evolution
  rules.
• docker-compose: rabbitmq with the management plugin (UI not published to the host), healthcheck,
  explicit dev credentials; prod profile fails fast without broker configuration.