Joins the weekly briefing list: email, topics and language, one record per email.

JSON or an HTML form. The request is checked (email, published topics) and stored once per email: asking again replaces the topics and the language. Before the first issue the list goes to CommsHarbor, which sends the confirmation (double opt-in) and the one-click unsubscribe. One subscription every 10 s per network: the rest of a flood gets the same answer and is not stored. inscricao.aberta in /api/briefings says whether it is open. Returns: { ok, status, message }