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>

src/web_interface/api_helpers.py

@@ -8,7 +8,6 @@ import time
 from typing import Any, Optional, Dict, Tuple
 from flask import jsonify, request
 
-from src.web_interface.error_handler import create_error_response, create_success_response
 from src.web_interface.errors import ErrorCode, WebInterfaceError
 
 
@@ -31,7 +30,15 @@ def success_response(
     Returns:
         Flask jsonify response
     """
-    response_data = create_success_response(data, message, metadata)
+    response_data: Dict[str, Any] = {'status': 'success'}
+    # `is not None` rather than truthiness: "" and {} are values a caller
+    # chose to send, and dropping them would make the shape depend on the data.
+    if data is not None:
+        response_data['data'] = data
+    if message is not None:
+        response_data['message'] = message
+    if metadata is not None:
+        response_data['metadata'] = metadata
     for key, value in (extra or {}).items():
         response_data.setdefault(key, value)
 
@@ -70,14 +77,14 @@ def error_response(
     Returns:
         Flask jsonify response with status code
     """
-    return create_error_response(
+    error = WebInterfaceError(
         error_code=error_code,
         message=message,
         details=details,
-        context=context,
-        suggested_fixes=suggested_fixes,
-        status_code=status_code
+        context=context or {},
+        suggested_fixes=suggested_fixes
     )
+    return jsonify(error.to_dict()), status_code
 
 
 def exception_error_response(