Switch to StatefulSet

I saw the official Helm chart the other day and one thing stood out — it models Weblate as a stateless Deployment rather than StatefulSet which is great for stateful services. As far as I know, Weblate is currently a stateful service and can't be scaled horizontally. We started using Weblate on Kubernetes way before the official Helm chart got released and we first modelled it as Deployment too but the upgrades were somewhat problematic and it kept failing when trying to re-attach persistent disk to the newly spun up container. We used the "Recreate" rollout strategy but it would still fail and then we switched over to StatefulSet and this issue's been gone ever since.

Anywya, the idea is, should we remodel Weblate as a StatefulService? Is there any specific reason why we're using the Deployment object? I'm assuming that you've already considered it and that there are some reasons that I might have not thought about.

Originally posted by @mareksuscak in https://github.com/WeblateOrg/weblate/discussions/4806

Start by reviewing the official Helm chart's current Deployment definition and the original discussion linked in the issue. Compare its upgrade and persistent-disk behavior with a StatefulSet, then determine whether the chart should be remodeled and what rationale or migration considerations must be documented; done means a decided approach and corresponding chart changes or an explicit reason to retain Deployment.