GET /api/sign/required_article returns one random article from the
"签到广告" article category (status=1, is_del=0, hidden=0). Empty
category or no articles returns null so clients can fall through to
direct sign-in. Powers the pre-sign-in reading gate per
docs/project-shaoyaoju/prd-require.md §6.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>