env(back-api): choose event bus solution

[spy4x]

Responsibilities of "event bus":

1. "Queue" A node posts an event, then a free node can grab and process it. Example: Delivery order statuses change: Order created -> Cooking -> Delivery in progress -> Delivered.

2. "Bus" A node posts an event, then all nodes are notified. Example: WebSocket notifications to all participants in a chat app.

Candidates:

• PostgreSQL
• Redis
• RabbitMQ and similar solutions
• Google Cloud Tasks? (better avoid vendor-locking)