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_web_api.py

@@ -551,7 +551,7 @@ class TestPluginsAPI:
         mock_plugin_catalog.get_all_plugin_info.return_value = [
             {'id': 'weather', 'name': 'Weather Plugin'}
         ]
-        api_v3.plugin_store_manager.get_registry_info.return_value = None
+        api_v3.plugin_store_manager.get_cached_registry_info.return_value = None
 
         response = client.get('/api/v3/plugins/installed')
 
@@ -571,7 +571,7 @@ class TestPluginsAPI:
             {'id': 'weather', 'name': 'Weather', 'version': '1.0.0'}
         ]
         # Registry advertises a newer version than the installed one.
-        api_v3.plugin_store_manager.get_registry_info.return_value = {
+        api_v3.plugin_store_manager.get_cached_registry_info.return_value = {
             'verified': True, 'latest_version': '1.2.0'
         }
 
@@ -592,7 +592,7 @@ class TestPluginsAPI:
         mock_plugin_catalog.get_all_plugin_info.return_value = [
             {'id': 'weather', 'name': 'Weather', 'version': '1.2.0'}
         ]
-        api_v3.plugin_store_manager.get_registry_info.return_value = {
+        api_v3.plugin_store_manager.get_cached_registry_info.return_value = {
             'verified': True, 'latest_version': '1.2.0'
         }