Use NodeBB JSON API instead of RSS for Opera release detection

Follow-up to #29649.

The Opera update-browser-releases script reads https://forums.opera.com/category/20.rss and filters by dc:creator to find threads authored by abitkulova. But RSS dc:creator is the latest poster, not the original author — so as soon as someone replies to a release thread, it drops out of the filter. That's what caused #29649 (#29649 patches the symptom with a numeric <= break; the fragility remains).

Proposal

Switch to the public NodeBB JSON API:

• GET /api/category/20 — lists topics with user.username = original poster, plus timestampISO (creation time, better release_date than RSS's last-reply pubDate).
• GET /api/topic/{tid}/{slug} — original post body as JSON, no HTML scraping needed for the engine version.

Fixes the reply-bumping problem, the HTML scraping, and the imprecise release date in one go.

/api/search requires auth, so we rely on the category listing. The NodeBB API isn't a documented contract — defensive parsing recommended.

Start by locating the update-browser-releases script and reading how it currently consumes the category RSS feed and extracts the engine version. Use the NodeBB category and topic JSON endpoints described in the issue, defensively parse their fields, and verify that replies do not change the detected author or release date.