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/widget_bundle.py

@@ -55,16 +55,10 @@ BUNDLE_ORDER = [
     "password-input.js",
     "timezone-selector.js",
     "plugin-loader.js",
-    # Reusable JSON file manager (used via x-widget: json-file-manager)
-    "json-file-manager.js",
 ]
 
 # Widget files that must NOT be bundled, with the reason.
-EXCLUDED = {
-    # Documentation example (docs/widget-guide.md); it registers the name
-    # 'color-picker' and would shadow the real color-picker.js.
-    "example-color-picker.js": "documentation example",
-}
+EXCLUDED = {}
 
 _lock = Lock()
 _cache = {"version": None, "body": None}