chore: remove dead code, deprecate unused plugin APIs (over-engineering audit)

Whole-tree audit. Every symbol was checked against core, the plugin
monorepo and all eight third-party plugins in plugins.json first.

- Deprecate (removal 3.10.0) plugin-facing methods nothing calls:
  LogoDownloader bulk download, ConfigManager backup/secret wrappers,
  APIHelper extras, BackgroundDataService poll API, PluginManager /
  PluginStateManager info readers, and a few CacheManager, FontManager,
  BaseOddsManager, DynamicTeamResolver methods and PluginTestCase.
  plugin_api_usage.py learns their receiver names; DEPRECATIONS doc
  regenerated.
- Remove core-internal dead code: CacheMetrics, Vegas status/stats
  plumbing, sync "new cycle" message (followers ignore unknown types),
  unused operation types, test-only PluginCatalog readers, IPC to_dict
  and ping, _parse_form_value, CacheStrategyProtocol, ErrorAggregator
  callbacks, duplicate web response helpers.
- Web UI: drop never-mounted json-file-manager.js, the example widget,
  utils/error_handler.js, four uncalled PluginAPI methods, and 29
  escapeHtml shims (call window.LEDEscape directly). Public globals,
  BaseWidget and widget names unchanged.
- Remove six one-off scripts (owner decision) and the unused markupsafe
  and pytest-mock pins.

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

web_interface/static/v3/js/plugins/api_client.js

@@ -219,55 +219,6 @@ const PluginAPI = {
         return [];
     },
     
-    /**
-     * Toggle plugin enabled/disabled.
-     * 
-     * @param {string} pluginId - Plugin identifier
-     * @param {boolean} enabled - Whether plugin should be enabled
-     * @returns {Promise<Object>} Response data
-     */
-    async togglePlugin(pluginId, enabled) {
-        return await this.request('/plugins/toggle', 'POST', {
-            plugin_id: pluginId,
-            enabled: enabled
-        });
-    },
-    
-    /**
-     * Get plugin configuration.
-     * 
-     * @param {string} pluginId - Plugin identifier
-     * @returns {Promise<Object>} Plugin configuration
-     */
-    async getPluginConfig(pluginId) {
-        const response = await this.request(`/plugins/config?plugin_id=${encodeURIComponent(pluginId)}`);
-        return response.data || {};
-    },
-    
-    /**
-     * Save plugin configuration.
-     * 
-     * @param {string} pluginId - Plugin identifier
-     * @param {Object} config - Configuration data
-     * @returns {Promise<Object>} Response data
-     */
-    async savePluginConfig(pluginId, config) {
-        return await this.request('/plugins/config', 'POST', {
-            plugin_id: pluginId,
-            config: config
-        });
-    },
-    
-    /**
-     * Reset plugin configuration to defaults.
-     * 
-     * @param {string} pluginId - Plugin identifier
-     * @returns {Promise<Object>} Response data
-     */
-    async resetPluginConfig(pluginId) {
-        return await this.request(`/plugins/config/reset?plugin_id=${encodeURIComponent(pluginId)}`, 'POST');
-    },
-    
     /**
      * Update plugin.
      *