3470

Added

• ArtworkMatchingService — New protocol-driven service (ArtworkMatchingServiceProtocol / ArtworkMatchingService) that encapsulates the progressive-fallback artwork search logic previously duplicated across ArtworkSearchQueue
• Box back artwork at import — ArtworkSearchQueue now requests both .boxFront and .boxBack in the primary pass and persists the back-cover image to game.boxBackArtworkURL when TheGamesDB returns one
• Background artwork tasks — Screenshot and title-screen artwork are fetched in a .background-priority Task after box art is saved, keeping the import pipeline non-blocking