* fix(threads): keep the owner row when deleting checkpoints fails
DELETE /api/threads/{id} treated the checkpoint delete as best-effort: a
failure was logged at DEBUG, the threads_meta row was removed anyway and
the request returned success. Read routes allow a thread without a meta
row as an untracked legacy thread, so the surviving checkpoints (title,
messages, goal) became readable by every signed-in user through
GET /threads/{id}, /state, /history and /goal, and the owner could not
retry because DELETE requires the row.
A failed checkpoint delete now logs the exception and returns a generic
500 before the meta row is touched, so the thread stays owned and the
owner can retry. The remaining cleanup steps stay best-effort; their
reads are owner-scoped.
* docs(changelog): reference #7992 for the thread delete checkpoint fix
* fix(threads): require checkpoint deletion before dropping ownership
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
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