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>

docs/MULTI_ROOT_WORKSPACE_SETUP.md

@@ -44,8 +44,8 @@ and symlink the plugin directories you are working on into LEDMatrix's
 ### 1. The plugin monorepo
 
 Clone ledmatrix-plugins into the same parent directory as LEDMatrix (the
-workspace file and `scripts/update_plugin_repos.py` look for
-`../ledmatrix-plugins` relative to the LEDMatrix root):
+workspace file looks for `../ledmatrix-plugins` relative to the LEDMatrix
+root):
 
 ```bash
 cd ~/Github
@@ -86,7 +86,7 @@ the plugin from there. See the
 
 ```bash
 cd ~/Github/LEDMatrix
-python3 scripts/update_plugin_repos.py      # git pull in ../ledmatrix-plugins
+git -C ../ledmatrix-plugins pull            # the sibling monorepo checkout
 # or
 ./scripts/dev/dev_plugin_setup.sh update    # git pull in every linked checkout
 ```