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_update_all.js

@@ -253,9 +253,10 @@ const noSleep = { sleep: async () => {} };
     for (const endpoint of bad) codes.push(await refusal(endpoint));
     ok('an endpoint that could leave the API path is refused before fetch()',
        codes.every(c => c === 'INVALID_ENDPOINT') && urls.length === 0, { codes, urls });
-    await PluginAPI.resetPluginConfig('a/../b&x=1');
+    global.debugLog = () => {};   // GETs go through the throttler, which logs
+    await PluginAPI.getPluginHealth('a/../b&x=1');
     ok('a plugin id is encoded into the URL, not spliced into it',
-       urls[0] === '/api/v3/plugins/config/reset?plugin_id=a%2F..%2Fb%26x%3D1', urls);
+       urls[0] === '/api/v3/plugins/health/a%2F..%2Fb%26x%3D1', urls);
     delete global.fetch;
   }