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>

server/nativeapi/config.go

@@ -16,13 +16,11 @@ import (
 // using partial masking (first and last character visible, middle replaced with *).
 // For values with 7+ characters: "secretvalue123" becomes "s***********3"
 // For values with <7 characters: "short" becomes "****"
-// Add field paths using dot notation (e.g., "LastFM.ApiKey", "Spotify.Secret")
+// Add field paths using dot notation (e.g., "LastFM.ApiKey")
 var sensitiveFieldsPartialMask = []string{
 	"LastFM.ApiKey",
 	"LastFM.Secret",
 	"Prometheus.MetricsPath",
-	"Spotify.ID",
-	"Spotify.Secret",
 	"DevAutoLoginUsername",
 }