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>

web_interface/templates/v3/base.html

@@ -456,10 +456,11 @@
         </div>
     </div>
 
-    <!-- Restart-pending banner: shown after a main-config save (display
-         hardware, rotation order, durations, general settings) — those only
-         take effect after the display service restarts. Plugin config saves
-         apply live and don't trigger this. Wired in app.js. -->
+    <!-- Restart-pending banner: shown when a response says restart_required
+         -- a main-config save (display hardware, rotation order, durations,
+         general settings), or a plugin install, update or uninstall the
+         running display cannot pick up live. Plugin config saves apply live
+         and don't trigger this. Wired in app.js (noteRestartRequired). -->
     <div id="restart-pending-banner" style="display:none"
          class="update-banner border-b transition-all duration-300 ease-in-out">
         <div class="mx-auto px-4 sm:px-6 lg:px-8 xl:px-12 2xl:px-16 py-2" style="max-width:100%">