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>

test/test_ipc_contract.py

@@ -60,7 +60,9 @@ class TestRoundTrip:
     def test_start_args_round_trip(self):
         args = OnDemandStartArgs(plugin_id='clock', mode='clock_main', duration=45.0,
                                  pinned=True)
-        assert OnDemandStartArgs.from_dict(_wire(args.to_dict())) == args
+        wire = _wire({'plugin_id': 'clock', 'mode': 'clock_main', 'duration': 45.0,
+                      'pinned': True})
+        assert OnDemandStartArgs.from_dict(wire) == args
 
     def test_encoded_messages_are_ascii_single_lines(self):
         data = c.encode_message({'v': 1, 'id': 'x', 'cmd': 'ping',