feat(cache): centralize absolute URL generation with PMO_SERVER_URL

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

pmoqobuz/src/api_rest.rs

@@ -299,7 +299,7 @@ async fn cache_album_image(mut album: Album, cover_cache: &Arc<pmocovers::Cache>
     if let Some(ref image_url) = album.image {
         match cover_cache.add_from_url(image_url, None).await {
             Ok(pk) => {
-                album.image_cached = Some(format!("/covers/images/{}", pk));
+                album.image_cached = Some(pmocache::covers_absolute_url_for(&pk, None));
             }
             Err(e) => {
                 tracing::warn!("Failed to cache album image: {}", e);