feat(music): add a stop command

Clears the whole queue and disconnects, mirrors the existing Stop
Manager function, just wires it up to the outbox like every other
dashboard command.

internal/music/listener.go

@@ -64,6 +64,9 @@ func dispatchCommand(cmd musicrepo.MusicCommand) error {
 	case "resume":
 		return Pause(guildID, false)
 
+	case "stop":
+		return Stop(guildID)
+
 	case "set_volume":
 		var payload struct {
 			Volume int `json:"volume"`