feat: switch on-demand tcp to persistent grpc channel for hi-prio messages

[gchiesa]

What

• Implemented persistent gRPC channels for high-priority messaging.
• Updated Docker Compose configuration to modify the number of replicas.

Why

• Persistent gRPC channels enhance the efficiency and reliability of high-priority message processing, reducing connection overhead.

Testing

• Unit tests added / updated (mise run tests passes)
• Linter passes (mise run lint passes)
• Manually tested with the Docker Compose stack (deployments/docker-compose/)
• No tests needed — explain why:

Checklist

• Tracking issue is referenced above
• Commit messages follow Conventional Commits format
• Assisted-By trailers added where an AI assistant contributed
• Documentation updated in docs/content/docs/ if behaviour changed