Context & Objective
The core implementation for the async step-down feature is already up. This ticket tracks the documentation of what we have built specifically for schema operations and KEK management.

Because these components interact with the lock-free synchronisation changes introduced by async step-down, we need to explicitly document their concurrency models. The goal is to validate the architectural assumptions made in the PR and provide a shared baseline to prevent subtle race conditions or ordering bugs in the future.

Scope
Focus strictly on schema operations (table creation and dropping) and KEK management workflows during an asynchronous step-down.