Board game tracking

Add board game tracking via the BoardGameGeek API (free, no key required). The domain already exists in the data model, pending implementation on the catalogue and screens side.

Backloggd (video games) has no official export yet — shouldn't be built against scraping.

Details

• Domain.BOARDGAMES is already declared, in premium early access (PREMIUM_DOMAINS), and boardgamegeek is already listed among the import sources (import-sources.ts), with no screen yet.
• BoardGameGeek's XML API is free and needs no key.
• Tracking model: "plays" are closer to dated game sessions than to a progress bar.