remove built-in Spotify integration (#5197)

* refactor: remove built-in Spotify integration

Remove the Spotify adapter and all related configuration, replacing
the built-in integration with the plugin system. This deletes the
adapters/spotify package, removes Spotify config options (ID/Secret),
updates the default agents list from "deezer,lastfm,spotify" to
"deezer,lastfm", and cleans up all references across configuration,
metrics, logging, artwork caching, and documentation. Users with
Spotify config options will now see a warning that the options are
no longer available.

* feat: add ListenBrainz to list of default agents

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

core/metrics/insights.go

@@ -199,7 +199,6 @@ var staticData = sync.OnceValue(func() insights.Data {
 	data.Config.EnableSharing = conf.Server.EnableSharing
 	data.Config.EnableStarRating = conf.Server.EnableStarRating
 	data.Config.EnableLastFM = conf.Server.LastFM.Enabled && conf.Server.LastFM.ApiKey != "" && conf.Server.LastFM.Secret != ""
-	data.Config.EnableSpotify = conf.Server.Spotify.ID != "" && conf.Server.Spotify.Secret != ""
 	data.Config.EnableListenBrainz = conf.Server.ListenBrainz.Enabled
 	data.Config.EnableDeezer = conf.Server.Deezer.Enabled
 	data.Config.EnableMediaFileCoverArt = conf.Server.EnableMediaFileCoverArt

core/metrics/insights/data.go

@@ -61,7 +61,6 @@ type Data struct {
 		EnableListenBrainz      bool   `json:"enableListenBrainz,omitempty"`
 		EnableDeezer            bool   `json:"enableDeezer,omitempty"`
 		EnableMediaFileCoverArt bool   `json:"enableMediaFileCoverArt,omitempty"`
-		EnableSpotify           bool   `json:"enableSpotify,omitempty"`
 		EnableJukebox           bool   `json:"enableJukebox,omitempty"`
 		EnablePrometheus        bool   `json:"enablePrometheus,omitempty"`
 		EnableCoverAnimation    bool   `json:"enableCoverAnimation,omitempty"`