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>

plugins/manifest_gen.go

@@ -57,7 +57,7 @@ type HTTPPermission struct {
 	Reason *string `json:"reason,omitempty" yaml:"reason,omitempty" mapstructure:"reason,omitempty"`
 
 	// List of required host patterns for HTTP requests (e.g., 'api.example.com',
-	// '*.spotify.com')
+	// '*.musicbrainz.org')
 	RequiredHosts []string `json:"requiredHosts,omitempty" yaml:"requiredHosts,omitempty" mapstructure:"requiredHosts,omitempty"`
 }
 
@@ -251,6 +251,6 @@ type WebSocketPermission struct {
 	Reason *string `json:"reason,omitempty" yaml:"reason,omitempty" mapstructure:"reason,omitempty"`
 
 	// List of required host patterns for WebSocket connections (e.g.,
-	// 'api.example.com', '*.spotify.com')
+	// 'api.example.com', '*.musicbrainz.org')
 	RequiredHosts []string `json:"requiredHosts,omitempty" yaml:"requiredHosts,omitempty" mapstructure:"requiredHosts,omitempty"`
 }