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/config_manager.py

@@ -45,6 +45,7 @@ from src.common.permission_utils import (
     ensure_shared_group_ownership,
     get_config_dir_mode
 )
+from src.deprecation import deprecated
 
 
 def _private_copy(config: Dict[str, Any]) -> Dict[str, Any]:
@@ -174,6 +175,7 @@ class ConfigManager:
         
         return result
     
+    @deprecated("3.10.0", "backups are handled by src.backup_manager")
     def rollback_config(self, backup_version: Optional[str] = None) -> bool:
         """
         Rollback configuration to a previous backup.
@@ -198,6 +200,7 @@ class ConfigManager:
         
         return success
     
+    @deprecated("3.10.0", "backups are handled by src.backup_manager")
     def list_backups(self) -> List[BackupInfo]:
         """
         List all available configuration backups.
@@ -208,6 +211,7 @@ class ConfigManager:
         atomic_mgr = self._get_atomic_manager()
         return atomic_mgr.list_backups()
     
+    @deprecated("3.10.0")
     def validate_config_file(self, config_path: Optional[str] = None) -> ValidationResult:
         """
         Validate a configuration file.
@@ -404,6 +408,7 @@ class ConfigManager:
             self.logger.error(error_msg, exc_info=True)
             raise ConfigError(error_msg, config_path=self.config_path) from e
 
+    @deprecated("3.10.0", "secrets are merged into each plugin's config; read them with config.get()")
     def get_secret(self, key: str) -> Optional[Any]:
         """Get a secret value by key."""
         try:
@@ -757,6 +762,7 @@ class ConfigManager:
             self.logger.error(error_msg, exc_info=True)
             raise ConfigError(error_msg, config_path=self.config_path, field=plugin_id) from e
     
+    @deprecated("3.10.0")
     def cleanup_orphaned_plugin_configs(self, valid_plugin_ids: List[str]) -> List[str]:
         """
         Remove configuration sections for plugins that are no longer installed.
@@ -810,6 +816,7 @@ class ConfigManager:
             self.logger.error(f"Error cleaning up orphaned plugin configs: {e}")
             return removed
     
+    @deprecated("3.10.0")
     def validate_all_plugin_configs(self, plugin_schema_manager=None) -> Dict[str, Dict[str, Any]]:
         """
         Validate all plugin configurations against their schemas.