## Why
Revert #27910 to remove the newly introduced thread `recencyAt`
persistence and API behavior from `main`.
## What changed
This reverts commit `fac3158c2a783095768076489815f361fa9b0db4`,
including the state migration, thread-store propagation, app-server API
surface, generated schemas, and related tests.
## Validation
Not run before opening; relying on CI for the initial fast signal.