refactor(web): read plugins through a PluginCatalog; only the display runs them (#688)

The web process built its own PluginManager and loaded plugins into itself:
store installs and updates loaded or reloaded a web-side copy, and config
saves and enable/disable called on_config_change, on_enable and on_disable
on it. None of that reached the panel, and /plugins/installed reported
runtime state from those copies.

- Add PluginCatalog (src/plugin_system/plugin_catalog.py): manifests,
  directories, display modes, installed version, schema and config reads,
  with no way to run a plugin. app.py and both blueprints use it; the
  plugin_manager blueprint attribute is gone.
- Remove every lifecycle call from the web routes. Config changes already
  reach the display through ConfigService (on_config_change) and the
  enabled-set reconcile.
- Health and metrics readers move to api_v3.health_tracker /
  resource_monitor. /plugins/installed reports loaded/state/error_info as
  null (the display does not publish them) and enabled by the display's
  rule.
- Store install, update and uninstall answer restart_required when the
  running display will not pick the change up by itself
  (display_restart_required). The restart banner follows the flag via
  window.noteRestartRequired instead of the /config/main URL heuristic;
  /config/main now sends restart_required: true.
- The one remaining in-process import of plugin code (Starlark helper
  modules, oauth_flow action scripts) goes through
  _import_plugin_code_in_web_process() until a web-entry contract.
- /plugins/installed reports vegas_participation (from #682) from the
  user's setting or the manifest, with vegas_participation_source; when
  only the plugin's code decides it, null with source 'runtime', since the
  web process no longer has plugin instances to ask.
- Check & Update All keeps its restart flags when the final list refresh
  fails, and asks for a restart when an enabled plugin's first request got
  no answer and the re-sent one found it up to date.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/web_interface/test_plugin_config_json_saves.py

@@ -72,7 +72,7 @@ STORED = {
     "skin_options": {"accent": "#00ff00"},
 }
 
-_ATTRS = ('config_manager', 'plugin_manager', 'plugin_store_manager',
+_ATTRS = ('config_manager', 'plugin_catalog', 'plugin_store_manager',
           'plugin_state_manager', 'saved_repositories_manager', 'schema_manager',
           'operation_queue', 'operation_history', 'cache_manager')
 
@@ -96,13 +96,12 @@ def env(tmp_path):
     plugin_manager = MagicMock()
     plugin_manager.plugin_manifests = {PLUGIN_ID: {"id": PLUGIN_ID}}
     plugin_manager.plugins_dir = plugins_dir
-    plugin_manager.get_plugin.return_value = None
 
     for name in _ATTRS:
         setattr(api_v3, name, MagicMock())
     api_v3.config_manager = config_manager
     api_v3.schema_manager = SchemaManager(plugins_dir=plugins_dir, project_root=tmp_path)
-    api_v3.plugin_manager = plugin_manager
+    api_v3.plugin_catalog = plugin_manager
     api_v3.operation_queue = None
 
     app = Flask(__name__)