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_render_cards.js

@@ -16,7 +16,7 @@ const container = {
   innerHTML: '',
   querySelectorAll: () => [],   // no skeletons in this harness
 };
-// escapeHtml() escapes via a detached element, so mirror what a browser does
+// LEDEscape.html() escapes via a detached element, so mirror what a browser does
 // when you read innerHTML back off textContent: & < > are escaped, quotes are not.
 class FakeEl {
   set textContent(v) { this._t = String(v == null ? '' : v); }
@@ -36,7 +36,7 @@ global.PLUGIN_DEBUG = false;
 global.debugLog = () => {};
 function setupInstalledEventDelegation() {}   // stubbed; tested separately
 
-eval(slice('function escapeHtml(text)', '\nfunction isNewPlugin'));
+eval(slice('function jsStringAttr(value)', '\nfunction isNewPlugin'));
 eval(slice('function renderInstalledCards(plugins, total)',
            '// Set up event delegation for plugin action buttons'));