Need better term than `state` for `singleton`, `queue` and `kv`

Currently, we use the term state in two different senses:
- to refer to the totality of all data the application works with, and
- to denote a specific part of the service that acts as a singleton, queue, and key-value storage.

When both meanings appear in the same context, it can be confusing. Therefore, we need a better name for the state-related components (singleton + queue + kv storage).

No files, tests, or entry points are named. First agree on the replacement term and identify the affected terminology; done when the singleton, queue, and key-value storage components have a consistent, documented name distinct from the application's overall state.

Written by the indexing model from the issue text.