chore: remove dead code, deprecate unused plugin APIs (over-engineering audit) (#783)

* 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>

* fix(web): calendar picker error text goes in a text node, not innerHTML

Same output as the escaped innerHTML it replaces; clears Codacy's
XSS-pattern alerts on the line.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/web_interface/errors.py

@@ -61,27 +61,14 @@ class WebInterfaceError:
     context: Optional[Dict[str, Any]] = None
     suggested_fixes: Optional[List[str]] = None
     original_error: Optional[Exception] = None
-    
-    def __init__(
-        self,
-        error_code: ErrorCode,
-        message: str,
-        details: Optional[str] = None,
-        context: Optional[Dict[str, Any]] = None,
-        suggested_fixes: Optional[List[str]] = None,
-        original_error: Optional[Exception] = None
-    ):
-        self.error_code = error_code
-        self.message = message
-        self.details = details
-        self.context = context or {}
+
+    def __post_init__(self) -> None:
+        self.context = self.context or {}
         # `is None`, not truthiness: an explicit [] means "this caller has
         # no suggestions to offer", which the default list would override.
-        self.suggested_fixes = (
-            suggested_fixes if suggested_fixes is not None
-            else self._get_default_suggestions(error_code))
-        self.original_error = original_error
-    
+        if self.suggested_fixes is None:
+            self.suggested_fixes = self._get_default_suggestions(self.error_code)
+
     def _get_default_suggestions(self, error_code: ErrorCode) -> List[str]:
         """Get default suggested fixes for error code."""
         suggestions_map = {