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/test_plugin_operation_queue.py

@@ -55,7 +55,7 @@ class TestPluginOperationQueue(unittest.TestCase):
         # behavior may differ. For this test, we'll verify the mechanism exists.
         try:
             self.queue.enqueue_operation(
-                OperationType.UPDATE,
+                OperationType.UNINSTALL,
                 "test-plugin"
             )
             # If no exception, the first operation may have completed
@@ -64,21 +64,6 @@ class TestPluginOperationQueue(unittest.TestCase):
             # Expected behavior - concurrent operation prevented
             pass
     
-    def test_operation_cancellation(self):
-        """Test cancelling a pending operation."""
-        operation_id = self.queue.enqueue_operation(
-            OperationType.INSTALL,
-            "test-plugin"
-        )
-        
-        # Cancel operation
-        success = self.queue.cancel_operation(operation_id)
-        self.assertTrue(success)
-        
-        # Check status
-        operation = self.queue.get_operation_status(operation_id)
-        self.assertEqual(operation.status, OperationStatus.CANCELLED)
-    
     def test_operation_history(self):
         """Test operation history tracking."""
         # Enqueue and complete an operation
@@ -92,7 +77,7 @@ class TestPluginOperationQueue(unittest.TestCase):
         time.sleep(0.5)
         
         # Check history
-        history = self.queue.get_operation_history(limit=10)
+        history = self.queue._operation_history
         self.assertGreater(len(history), 0)
         
         # Find our operation in history