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/select-dropdown.js

@@ -28,8 +28,6 @@
 
     const base = window.BaseWidget ? new window.BaseWidget('SelectDropdown', '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, '_');
@@ -73,7 +71,7 @@
 
             html += `
                 <select id="${fieldId}_input"
-                        name="${escapeHtml(options.name || fieldId)}"
+                        name="${window.LEDEscape.html(options.name || fieldId)}"
                         ${disabled ? 'disabled' : ''}
                         ${required ? 'required' : ''}
                         onchange="window.LEDMatrixWidgets.getHandlers('select-dropdown').onChange('${fieldId}')"
@@ -82,14 +80,14 @@
 
             // Placeholder option
             if (placeholder && !required) {
-                html += `<option value="" ${!currentValue ? 'selected' : ''}>${escapeHtml(placeholder)}</option>`;
+                html += `<option value="" ${!currentValue ? 'selected' : ''}>${window.LEDEscape.html(placeholder)}</option>`;
             }
 
             // Options
             for (const optValue of enumValues) {
                 const label = labels[optValue] || String(optValue).replace(/_/g, ' ').replace(/\b\w/g, l => l.toUpperCase());
                 const isSelected = String(optValue) === currentValue;
-                html += `<option value="${escapeHtml(String(optValue))}" ${isSelected ? 'selected' : ''}>${escapeHtml(label)}</option>`;
+                html += `<option value="${window.LEDEscape.html(String(optValue))}" ${isSelected ? 'selected' : ''}>${window.LEDEscape.html(label)}</option>`;
             }
 
             html += '</select>';