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-schema.json

@@ -149,7 +149,7 @@
         },
         "requiredHosts": {
           "type": "array",
-          "description": "List of required host patterns for HTTP requests (e.g., 'api.example.com', '*.spotify.com')",
+          "description": "List of required host patterns for HTTP requests (e.g., 'api.example.com', '*.musicbrainz.org')",
           "items": {
             "type": "string"
           }
@@ -189,7 +189,7 @@
         },
         "requiredHosts": {
           "type": "array",
-          "description": "List of required host patterns for WebSocket connections (e.g., 'api.example.com', '*.spotify.com')",
+          "description": "List of required host patterns for WebSocket connections (e.g., 'api.example.com', '*.musicbrainz.org')",
           "items": {
             "type": "string"
           }