core: properly dispose of sqlalchemy engine

otherwise for some reason this was causing shm/wal turnds (and test_cache_paths to fail)

the commit that 'broke' it is sqlalchemy/sqlalchemy@22ed657 -- not sure what's up

src/cachew/__init__.py

@@ -599,6 +599,7 @@ def __enter__(self) -> 'DbHelper':
 
     def __exit__(self, *args) -> None:
         self.connection.close()
+        self.engine.dispose()
 
 
 HashFunction = Callable[..., SourceHash]