fix: migrate legacy state to the first Discord server

tests/test_guild_config_repository.py

@@ -14,6 +14,7 @@ class GuildConfigRepositoryTest(unittest.TestCase):
             path.write_text(json.dumps({"default_plays": 7, "10": 123}), encoding="utf-8")
             repository = GuildConfigRepository(JsonFileStore(str(path)), fallback_plays=5)
 
+            repository.migrate_legacy(10)
             self.assertEqual(repository.get_default_plays(10), 7)
             self.assertEqual(repository.get_notification_channel_id(10), 123)
             self.assertEqual(repository.get_default_plays(20), 5)