refactor: rename check_migrations → is_migrated

Boolean-returning methods should read as predicates.
Renames DbStore.check_migrations, StoreManager.check_migrations,
and the test patch accordingly.

tests/unit/test_cli_dispatch.py

@@ -20,7 +20,7 @@ from bot_bottle.store_manager import StoreManager
 
 class TestMainDispatch(unittest.TestCase):
     def setUp(self) -> None:
-        patcher = patch.object(DbStore, "check_migrations", return_value=True)
+        patcher = patch.object(DbStore, "is_migrated", return_value=True)
         self._mock_check = patcher.start()
         self.addCleanup(patcher.stop)
         self.addCleanup(StoreManager.reset)