Migrate staging deployments to Kubernetes

Staging deployments still depend on Docker Compose setup. To move the staging environment closer to the production instance it should use the same deployment type.

This change would also unblock Docker Compose-based setup refactoring.

Start by locating the staging deployment configuration and the current Docker Compose setup, then compare it with the production deployment type. Done means staging runs on Kubernetes like production and the Docker Compose-based setup can be refactored afterward.

索引モデルが issue の本文から書いたものです。