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/web_interface/integration/test_plugin_operations.py

@@ -66,31 +66,6 @@ class TestPluginOperationsIntegration(unittest.TestCase):
         history = self.operation_history.get_history(plugin_id=plugin_id)
         self.assertEqual([r.operation_type for r in history], ["install"])
 
-    def test_update_operation_flow(self):
-        """Test complete update operation flow."""
-        plugin_id = "test-plugin"
-        
-        # Enqueue update operation
-        operation_id = self.operation_queue.enqueue_operation(
-            OperationType.UPDATE,
-            plugin_id,
-            {"from_version": "1.0.0", "to_version": "2.0.0"}
-        )
-        
-        self.assertIsNotNone(operation_id)
-        
-        # Record in history
-        self.operation_history.record_operation(
-            operation_type="update",
-            plugin_id=plugin_id,
-            status="in_progress",
-            operation_id=operation_id
-        )
-        
-        # Verify history
-        history = self.operation_history.get_history(plugin_id=plugin_id)
-        self.assertEqual([r.operation_type for r in history], ["update"])
-
     def test_uninstall_operation_flow(self):
         """Test complete uninstall operation flow."""
         plugin_id = "test-plugin"
@@ -162,7 +137,7 @@ class TestPluginOperationsIntegration(unittest.TestCase):
         # The prevention only works for truly concurrent (pending/running) operations
         try:
             op2_id = self.operation_queue.enqueue_operation(
-                OperationType.UPDATE,
+                OperationType.UNINSTALL,
                 plugin_id
             )
             # If no exception, the first operation may have completed already