fix(web): the installed-plugins list never waits on the registry download (#766)

GET /plugins/installed called get_registry_info() per plugin. Despite the
"no network call" comment, a cold or expired cache made that download
plugins.json (10 s timeout, three attempts), and with no cached copy each
plugin's lookup repeated it -- offline, every load waited out the timeouts.

The route now reads the registry copy already in memory, however old, via
get_cached_registry_info(). A missing or expired copy starts a single
background refresh (backing off after an offline failure), so a later load
gets update and verified badges. Store, install and update paths still fetch.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

test/test_vegas_participation.py

@@ -446,7 +446,7 @@ class TestInstalledPluginsApi:
                     **(manifest_extra or {})}
             api.plugin_catalog.get_all_plugin_info = MagicMock(return_value=[info])
             api.plugin_catalog.get_plugin_directory = MagicMock(return_value=None)
-            api.plugin_store_manager.get_registry_info = MagicMock(return_value=None)
+            api.plugin_store_manager.get_cached_registry_info = MagicMock(return_value=None)
             api.config_manager.load_config = MagicMock(return_value={'demo': config})
             response = api_v3_client.get('/api/v3/plugins/installed')
             assert response.status_code == 200