Replace hardcoded relative URLs and manual base_url concatenation with a unified absolute URL API via pmocache::covers_absolute_url_for() and CacheTrait::absolute_url_for().
- Add pmocache as a required dependency to pmoparadise
- Introduce absolute_url_for() and covers_absolute_url_for() helpers using PMO_SERVER_URL env var (default: http://localhost:8080)
- Update all callers to use absolute URLs for covers and audio in streaming, playlists, Qobuz, Radio France, UPnP, and server startup
- Remove redundant route_for() usage in URL construction
- Add pmocache to Cargo.lock