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/plugin_system/store_registry.py

@@ -183,16 +183,7 @@ class _RegistryMixin:
     @staticmethod
     def _distinct_sequence(values: List[str]) -> List[str]:
         """Return list preserving order while removing duplicates and falsey entries."""
-        seen = set()
-        ordered = []
-        for value in values:
-            if not value:
-                continue
-            if value in seen:
-                continue
-            seen.add(value)
-            ordered.append(value)
-        return ordered
+        return list(dict.fromkeys(v for v in values if v))
 
     def _validate_manifest_version_fields(self, manifest: Dict[str, Any]) -> List[str]:
         """