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>

web_interface/app.py

@@ -159,11 +159,7 @@ schema_manager = SchemaManager(
 # saves reach the running plugins through the display's config watcher; what
 # the display knows at run time (health, metrics, errors, current mode) it
 # publishes to the shared cache. See docs/ARCHITECTURE.md.
-plugin_catalog = PluginCatalog(
-    plugins_dir=plugins_dir,
-    config_manager=config_manager,
-    schema_manager=schema_manager,
-)
+plugin_catalog = PluginCatalog(plugins_dir=plugins_dir)
 
 # Initialize operation queue for plugin operations
 operation_queue = PluginOperationQueue(max_history=500)
@@ -173,12 +169,10 @@ operation_queue = PluginOperationQueue(max_history=500)
 # snapshot the display publishes (src/plugin_system/plugin_runtime.py). An
 # existing file is left where it is, unread; see docs/ARCHITECTURE.md.
 
-# Initialize operation history
-# Use lazy_load=True to defer file loading until first use (improves startup time)
+# Initialize operation history (its file is read on first use, not at startup)
 operation_history = OperationHistory(
     history_file=str(project_root / "data" / "operation_history.json"),
-    max_records=1000,
-    lazy_load=True
+    max_records=1000
 )
 
 # Plugin discovery is deferred until first API request that needs it