Option to auto-delete threads untouched for N days

[moapu]

Settings → Storage can clean up old worktrees, logs and browser artifacts by age, but not threads. Could we get the same for threads?

Delete threads inactive for [N] days: off by default, N set by the user. Pinned threads and threads with work in progress are skipped.

For this to actually shrink the database, deleting a thread would need to remove its history, which it doesn't today (#17845).