Submit thumbs-up or thumbs-down feedback for a profile returned by a search.

Use the chat_thread_id from the search response together with the profile
linkedin_slug. reason is optional and is typically provided for downvotes.

Ratings are keyed on the chat thread, never on the per-call uuid of a search
response: reopening a thread serves the cached ranking but returns a new uuid,
so keying on it would lose the feedback. Pass that per-call value as the optional
search_call_uuid to record which ranking was rated.

A rating belongs to the thread rather than to its author: there is one rating per
(chat_thread_id, linkedin_slug), and re-submitting updates it even when a
different user submits it.

An existing reason is kept when the rating is unchanged and omitted, and cleared when the rating changes.