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>

docs/PLUGIN_API_REFERENCE.md

@@ -149,7 +149,11 @@ Clean up resources when plugin is unloaded. Override to close connections, stop
 
 #### `on_config_change(new_config: Dict[str, Any]) -> None`
 
-Called after plugin configuration is updated via web API.
+Called after the plugin's section of `config.json` changes -- a save in the
+web UI, say. Every lifecycle hook runs in the display process, which is the
+only process that runs plugins: the web interface writes `config.json`, and
+the display's config watcher calls this with the prepared section. See
+[ARCHITECTURE.md](ARCHITECTURE.md#web-and-display-processes-who-runs-plugins).
 
 In the display service it runs on the config watcher thread while holding
 the plugin's lock, so it never overlaps your `update()` or `display()`. If
@@ -159,11 +163,13 @@ from the update thread: as soon as the plugin is free, and before its next
 
 #### `on_enable() -> None`
 
-Called when plugin is enabled.
+Called when the display loads the plugin enabled: at startup, or when it is
+switched on in the web UI.
 
 #### `on_disable() -> None`
 
-Called when plugin is disabled.
+Called when the display unloads the plugin, e.g. when it is switched off in
+the web UI.
 
 #### `get_update_interval() -> Optional[float]`