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/README.md

@@ -28,7 +28,7 @@ set by `BUNDLE_ORDER` there.
 | `file-upload-single` | string | One image upload; stores the uploaded file's relative path |
 | `google-oauth` | string | Step 2 of the calendar plugin's Google sign-in |
 | `plugin-file-manager` | null | Inline file manager driven by the plugin's `web_ui_actions` |
-| `json-file-manager` | null | JSON data-file manager driven by `web_ui_actions` |
+| `json-file-manager` | null | Embeds the plugin's own `web_ui/file_manager.html` in an iframe |
 | `toggle-switch` | boolean | On/off switch |
 | `slider` | integer / number | Range slider using `minimum` / `maximum` |
 | `number-input` | integer / number | Number field with min/max check |
@@ -51,7 +51,6 @@ Other files here:
 | `notification.js` | Toast notifications; owns `window.showNotification` |
 | `plugin-order-list.js` | Drag-and-drop plugin order list used by the Display and Durations tabs (`window.PluginOrderList`) |
 | `plugin-loader.js` | Loads a plugin-supplied widget on demand |
-| `example-color-picker.js` | Example custom widget. Not bundled: it registers `color-picker` and would replace the real one |
 
 Each widget file's header comment gives its schema options. The sections
 below cover the ones that need more than a line.
@@ -270,8 +269,6 @@ directory is the only place the core serves plugin widgets from.
 })();
 ```
 
-[`example-color-picker.js`](example-color-picker.js) is a longer example.
-
 ### 2. Reference it in the schema
 
 ```json