Use /api/v1 with token authentication and typed responses

Closes #38, #39 and #40; adds the client side of #41.

- Every request goes to /api/v1. MusicServerClient gets a token with
  Settings.api_password the first time one is needed, reuses it, and on a
  401 gets a new one and sends the request once more. Switching server gets
  a new token too. Callers never see the token.
- Sync is started with POST, as MusicServer 5.0 requires.
- The client has every endpoint in the server's OpenAPI spec, including
  statistics, soundtracks and songs by UUID, version, downloads and tokens.
- tools/generate_api_types.gd generates a typed class per schema in
  server/openapi.json into server/generated/. Song info, statistics and
  versions are passed to callers as these classes. A list the server sends
  as null is an empty list.
- FakeHttpTransport hands out tokens and answers 401 without one, so every
  test goes through the token flow. New tests cover the token handling,
  every endpoint and each generated class.

quiz/song_loader.gd

@@ -22,19 +22,16 @@ var retry_seconds: float = 5.0
 ## Preloading waits for the list, because loading the list clears the playlist.
 func load_song_list() -> void:
 	GameLog.debug("load_song_list")
-	var song_list_fetched: Callable = func song_list_fetched(data: Variant) -> void:
+	var song_list_fetched: Callable = func song_list_fetched(songs: Array[SongInfo]) -> void:
 		Playlist.clear_song_list()
-		if typeof(data) == TYPE_ARRAY:
-			for d: Dictionary in data:
-				var new_song_object: SongObject = SongObject.new()
-				new_song_object.song_number = d.SongNo
-				new_song_object.soundtrack_title = d.Soundtrack
-				new_song_object.song_title = d.Song
-				new_song_object.is_answered = true
-				new_song_object.has_played = true
-				Playlist.add_to_playlist(new_song_object)
-		elif data != null:
-			GameLog.warning("Unexpected data")
+		for info: SongInfo in songs:
+			var new_song_object: SongObject = SongObject.new()
+			new_song_object.song_number = info.song_no
+			new_song_object.soundtrack_title = info.soundtrack
+			new_song_object.song_title = info.song
+			new_song_object.is_answered = true
+			new_song_object.has_played = true
+			Playlist.add_to_playlist(new_song_object)
 		Playlist.update_last_played_song()
 		song_list_loaded.emit()
 		fetch_song_if_idle()
@@ -80,11 +77,11 @@ func _add_song_to_queue(new_song_object: SongObject) -> void:
 
 
 func _fetch_song_info(new_song_object: SongObject) -> void:
-	var song_info_fetched: Callable = func song_info_fetched(data_received: Dictionary) -> void:
+	var song_info_fetched: Callable = func song_info_fetched(info: SongInfo) -> void:
 		GameLog.debug("song_info_fetched")
-		new_song_object.song_number = data_received.SongNo
-		new_song_object.soundtrack_title = data_received.Soundtrack
-		new_song_object.song_title = data_received.Song
+		new_song_object.song_number = info.song_no
+		new_song_object.soundtrack_title = info.soundtrack
+		new_song_object.song_title = info.song
 		GameLog.debug("new_song_object: ", new_song_object.to_string())
 		Playlist.add_to_playlist(new_song_object)
 		if Playlist.should_preload_more_songs():