Read Redditi Session

The caller's saved progress through the Redditi PF walkthrough (AP-071).

tax_year is required, unlike the year on /incassi, /estimate and /deadlines. Those describe the year that is currently running, so the server's clock is a defensible default; a return is filed for a year that has finished, so defaulting to today's year would silently open a session against the wrong filing — and the client always knows which return the user opened.

204 rather than 404 for a year with no row: the wizard renders that as a fresh walkthrough, and 404 is already this router's "the feature is off" signal.