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

pmoparadise/Cargo.toml

@@ -63,6 +63,7 @@ pmoplaylist = { path = "../pmoplaylist" }
 pmoconfig = { path = "../pmoconfig", optional = true }
 
 # Cache support (OBLIGATOIRE - architecture refactorisée)
+pmocache = { path = "../pmocache" }
 pmocovers = { path = "../pmocovers" }
 pmoaudiocache = { path = "../pmoaudiocache" }