Get Latest Briefing

Return the user's current briefing, or None if there isn't a fresh one.

Only briefings covering today or yesterday qualify; anything older is history, not "this morning".

Stored content that fails to validate against the current BriefingContent shape (e.g. written by an older/newer version of the composer) is skipped rather than 500ing the request — and rather than hiding every older briefing behind one unreadable row on the newest date.