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/toggle-switch.js

@@ -26,8 +26,6 @@
 
     const base = window.BaseWidget ? new window.BaseWidget('ToggleSwitch', '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, '_');
@@ -90,10 +88,10 @@
             const sizeClass = SIZE_CLASSES[size] || SIZE_CLASSES.medium;
             const colorClass = COLOR_CLASSES[colorOn] || COLOR_CLASSES.blue;
 
-            let html = `<div id="${fieldId}_widget" class="toggle-switch-widget flex items-center" data-field-id="${fieldId}" data-label-on="${escapeHtml(labelOn)}" data-label-off="${escapeHtml(labelOff)}" data-color="${colorOn}">`;
+            let html = `<div id="${fieldId}_widget" class="toggle-switch-widget flex items-center" data-field-id="${fieldId}" data-label-on="${window.LEDEscape.html(labelOn)}" data-label-off="${window.LEDEscape.html(labelOff)}" data-color="${colorOn}">`;
 
             // Hidden checkbox for form submission
-            html += `<input type="hidden" id="${fieldId}_hidden" name="${escapeHtml(options.name || fieldId)}" value="${isChecked}">`;
+            html += `<input type="hidden" id="${fieldId}_hidden" name="${window.LEDEscape.html(options.name || fieldId)}" value="${isChecked}">`;
 
             html += `
                 <button type="button"
@@ -113,7 +111,7 @@
             if (showLabels) {
                 html += `
                     <span id="${fieldId}_label" class="ml-3 text-sm font-medium ${isChecked ? 'text-gray-900' : 'text-gray-500'}">
-                        ${escapeHtml(isChecked ? labelOn : labelOff)}
+                        ${window.LEDEscape.html(isChecked ? labelOn : labelOff)}
                     </span>
                 `;
             }