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/static/v3/js/widgets/textarea.js

@@ -26,8 +26,6 @@
 
     const base = window.BaseWidget ? new window.BaseWidget('Textarea', '1.0.0') : null;
 
-    function escapeHtml(text) { return window.LEDEscape.html(text); }
-
     function sanitizeId(id) {
         if (base) return base.sanitizeId(id);
         return String(id).replace(/[^a-zA-Z0-9_-]/g, '_');
@@ -75,15 +73,15 @@
 
             html += `
                 <textarea id="${fieldId}_input"
-                          name="${escapeHtml(options.name || fieldId)}"
+                          name="${window.LEDEscape.html(options.name || fieldId)}"
                           rows="${rows}"
-                          placeholder="${escapeHtml(placeholder)}"
+                          placeholder="${window.LEDEscape.html(placeholder)}"
                           ${maxLength ? `maxlength="${maxLength}"` : ''}
                           ${minLength ? `minlength="${minLength}"` : ''}
                           ${disabled ? 'disabled' : ''}
                           onchange="window.LEDMatrixWidgets.getHandlers('textarea').onChange('${fieldId}')"
                           oninput="window.LEDMatrixWidgets.getHandlers('textarea').onInput('${fieldId}')"
-                          class="form-textarea w-full rounded-md border-gray-300 shadow-sm focus:border-blue-500 focus:ring-blue-500 ${resizeClass} ${disabled ? 'bg-gray-100 cursor-not-allowed' : 'bg-white'} text-black placeholder:text-gray-400">${escapeHtml(currentValue)}</textarea>
+                          class="form-textarea w-full rounded-md border-gray-300 shadow-sm focus:border-blue-500 focus:ring-blue-500 ${resizeClass} ${disabled ? 'bg-gray-100 cursor-not-allowed' : 'bg-white'} text-black placeholder:text-gray-400">${window.LEDEscape.html(currentValue)}</textarea>
             `;
 
             // Character count