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>

test/js/unit/test_inline_handler_escaping.js

@@ -62,12 +62,12 @@ global.setGridHtmlIfChanged = (container, html) => { container.innerHTML = html;
 
 // eslint-disable-next-line no-eval
 eval([
-  'function escapeHtml(text) {', 'function escapeAttribute(text) {', 'function jsStringAttr(value) {',
+  'function jsStringAttr(value) {',
   'function renderPluginStore(plugins) {', 'function renderSavedRepositories(repositories) {',
   'function renderCustomRegistryPlugins(plugins, registryUrl) {',
-].map(extract).join('\n') + '\nglobal.jsStringAttr = jsStringAttr; global.escapeHtml = escapeHtml;'
+].map(extract).join('\n') + '\nglobal.jsStringAttr = jsStringAttr;'
   + '\nglobal.renderPluginStore = renderPluginStore; global.renderSavedRepositories = renderSavedRepositories;'
-  + '\nglobal.renderCustomRegistryPlugins = renderCustomRegistryPlugins; global.escapeAttribute = escapeAttribute;');
+  + '\nglobal.renderCustomRegistryPlugins = renderCustomRegistryPlugins;');
 
 // ── minimal HTML start-tag tokenizer ───────────────────────────────────────
 function decodeEntities(s) {