Write Redditi Session

Save one step of that progress and hand back the row as stored.

The response is what the database returned, not the request echoed: completed_at is decided there (COALESCE against what is already stored), so a second "done" keeps the first completion's timestamp and reopening the walkthrough clears it. A client that trusted its own payload would report the wrong completion time for every save after the first.

No profile gate, unlike /estimate and /deadlines: those compute regime-specific money, while this stores progress markers — which rigo is ticked off — and refusing to remember them until a profile is complete would make the walkthrough unusable for exactly the people using it to find out what their profile should say. The year gate stays, because a session for a year with no verified parameters could never be filled in.