fix(thumbnails): keep the video fallback tested and stop the preview worker before cleanup

A video listed without a preview still renders its own element and is
released on refresh. Test teardowns and the browser reset stop the
automatic preview worker before the temp tree goes, so no preview lands
after cleanup (Windows). TESTING.md documents automatic previews.

tests/test_api_admin.py

@@ -79,6 +79,8 @@ class AdminAPITestCase(AioHTTPTestCase):
 
     async def tearDownAsync(self):
         await super().tearDownAsync()  # closes the aiohttp test client
+        # The automatic preview worker writes into the temp tree: stop it first.
+        self.api.close_auto_thumbnails()
         GalleryConfig.MONITORING_DIRECTORIES = self._orig_gallery_dirs
         PromptManagerConfig.update_config(self._orig_pm_config)
         for key, value in self._orig_env.items():