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/test_onboarding_checklist.py

@@ -57,7 +57,7 @@ def render(config):
     # pages_v3 is a module-level singleton shared across the test process;
     # restore whatever the previous test left on it.
     original_cm = getattr(pv.pages_v3, "config_manager", None)
-    original_pm = getattr(pv.pages_v3, "plugin_manager", None)
+    original_pm = getattr(pv.pages_v3, "plugin_catalog", None)
 
     mock_cm = MagicMock()
     mock_cm.load_config.return_value = config
@@ -65,10 +65,9 @@ def render(config):
     pv.pages_v3.config_manager = mock_cm
 
     mock_pm = MagicMock()
-    mock_pm.plugins = {}
     mock_pm.get_all_plugin_info.return_value = []
     mock_pm.get_plugin_display_modes.side_effect = lambda pid: []
-    pv.pages_v3.plugin_manager = mock_pm
+    pv.pages_v3.plugin_catalog = mock_pm
 
     app.register_blueprint(pv.pages_v3, url_prefix="")
     try:
@@ -77,7 +76,7 @@ def render(config):
         return resp.get_data(as_text=True)
     finally:
         pv.pages_v3.config_manager = original_cm
-        pv.pages_v3.plugin_manager = original_pm
+        pv.pages_v3.plugin_catalog = original_pm
 
 
 def timezone_step(body):