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/requirements.txt

@@ -8,7 +8,6 @@ werkzeug>=3.1.6,<4.0.0  # Flask transitive; pinned to keep a security floor abov
 flask-limiter>=3.5.0,<4.0.0  # Rate limiting (prevent accidental abuse)
 flask-compress>=1.14  # gzip/brotli response compression (big win for the large JS/HTML over WiFi)
 jinja2>=3.1.6,<4.0.0  # Flask transitive, but imported directly (TemplateNotFound); 3.1.6 is the security floor — earlier 3.1.x has sandbox breakouts
-markupsafe>=2.1.0,<4.0.0  # Flask transitive, but imported directly (escape)
 
 # WebSocket support: intentionally NOT declared here. The web interface
 # uses Server-Sent Events, and plugins that need Socket.IO (e.g.