Set Process Conversation Model

Persist the per-conversation model choice so it survives reload and syncs across devices. Upserts ONLY the provider/model columns on the conversation cache row — every other column (history/meta/route/…) is untouched, so a concurrent core→cache trajectory sync won't clobber the model and this PUT won't clobber history. If the row doesn't exist yet (model picked before the first trajectory sync created it), a minimal row is inserted; the later sync fills in the rest via its own upsert, which deliberately omits these two columns.