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/plugin_manager.py

@@ -38,6 +38,7 @@ from src.common.permission_utils import (
     ensure_directory_permissions,
     get_plugin_dir_mode
 )
+from src.deprecation import deprecated
 
 
 class _DeferredConfigChange(NamedTuple):
@@ -939,6 +940,7 @@ class PluginManager:
         """
         return self.plugins.get(plugin_id)
     
+    @deprecated("3.10.0", "use get_plugin(plugin_id)")
     def get_all_plugins(self) -> Dict[str, Any]:
         """
         Get all loaded plugins.
@@ -948,6 +950,7 @@ class PluginManager:
         """
         return self.plugins.copy()
     
+    @deprecated("3.10.0", "read the manifest with src.plugin_system.plugin_catalog.PluginCatalog")
     def get_plugin_info(self, plugin_id: str) -> Optional[Dict[str, Any]]:
         """
         Get information about a plugin (manifest + runtime info).
@@ -985,6 +988,7 @@ class PluginManager:
         
         return info
     
+    @deprecated("3.10.0", "read manifests with src.plugin_system.plugin_catalog.PluginCatalog")
     def get_all_plugin_info(self) -> List[Dict[str, Any]]:
         """
         Get information about all plugins.
@@ -1025,6 +1029,7 @@ class PluginManager:
             by_manifest=False)
         return str(plugin_dir) if plugin_dir is not None else None
     
+    @deprecated("3.10.0", "read manifests with src.plugin_system.plugin_catalog.PluginCatalog")
     def get_plugin_display_modes(self, plugin_id: str) -> List[str]:
         """
         Get display modes provided by a plugin.
@@ -1045,6 +1050,7 @@ class PluginManager:
             return display_modes
         return []
     
+    @deprecated("3.10.0", "read manifests with src.plugin_system.plugin_catalog.PluginCatalog")
     def find_plugin_for_mode(self, mode: str) -> Optional[str]:
         """
         Find which plugin provides a given display mode.