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_operation_queue_pending_and_trim.py

@@ -54,7 +54,7 @@ def test_second_pending_operation_for_a_plugin_is_refused(op_queue):
     assert _wait_for(lambda: op_queue.get_operation_status(first).status
                      == OperationStatus.COMPLETED)
     # Once it has finished, the plugin accepts a new operation again.
-    op_queue.enqueue_operation(OperationType.UPDATE, "demo")
+    op_queue.enqueue_operation(OperationType.UNINSTALL, "demo")
 
 
 def test_operations_map_is_trimmed_with_history(op_queue):
@@ -64,8 +64,8 @@ def test_operations_map_is_trimmed_with_history(op_queue):
     assert _wait_for(lambda: all(
         (op_queue.get_operation_status(i) is None
          or op_queue.get_operation_status(i).status == OperationStatus.COMPLETED)
-        for i in ids) and len(op_queue.get_operation_history()) == 3)
+        for i in ids) and len(op_queue._operation_history) == 3)
 
     assert len(op_queue._operations) == 3
-    kept = {op.operation_id for op in op_queue.get_operation_history()}
+    kept = {op.operation_id for op in op_queue._operation_history}
     assert set(op_queue._operations) == kept