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>

requirements-test.txt

@@ -2,7 +2,6 @@
 # Install alongside requirements.txt:  pip install -r requirements.txt -r requirements-test.txt
 pytest>=9.0.3,<10.0.0
 pytest-cov>=4.1.0,<8.0.0
-pytest-mock>=3.11.0,<4.0.0
 freezegun>=1.2,<2        # deterministic time for golden-image tests
 psutil>=6.0.0,<7.0.0     # optional at runtime; installed for tests so the
                          # /system/status endpoint's real path is exercised