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>

src/plugin_system/store_manager.py

@@ -138,6 +138,11 @@ class PluginStoreManager(_RegistryMixin, _InstallMixin, _UpdateMixin):
         # the registry cache expires. Only one thread fetches; others wait and
         # then get the result from the warm cache (double-checked locking).
         self._registry_fetch_lock = threading.Lock()
+        # refresh_registry_in_background: the one refresh thread, and when
+        # an offline one may be retried (see that method).
+        self._registry_refresh_lock = threading.Lock()
+        self._registry_refresh_thread: Optional[threading.Thread] = None
+        self._registry_refresh_retry_after = 0.0
 
         # Per-plugin locks for _reinstall_with_rollback: the web UI runs
         # Flask with threaded=True, so two overlapping requests for the