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_state.py

@@ -15,6 +15,7 @@ from datetime import datetime
 import logging
 
 from src.logging_config import get_logger
+from src.deprecation import deprecated
 
 
 class PluginState(Enum):
@@ -138,6 +139,7 @@ class PluginStateManager:
         """
         return self._states.get(plugin_id, PluginState.UNLOADED)
     
+    @deprecated("3.10.0", "use get_state()")
     def is_loaded(self, plugin_id: str) -> bool:
         """Check if plugin is loaded."""
         state = self.get_state(plugin_id)
@@ -148,11 +150,13 @@ class PluginStateManager:
         state = self.get_state(plugin_id)
         return state == PluginState.ENABLED
     
+    @deprecated("3.10.0", "use get_state()")
     def is_running(self, plugin_id: str) -> bool:
         """Check if plugin is currently running."""
         state = self.get_state(plugin_id)
         return state == PluginState.RUNNING
     
+    @deprecated("3.10.0", "use get_state()")
     def is_error(self, plugin_id: str) -> bool:
         """Check if plugin is in error state."""
         state = self.get_state(plugin_id)
@@ -197,6 +201,7 @@ class PluginStateManager:
                 state.value,
             )
 
+    @deprecated("3.10.0")
     def get_error_info(self, plugin_id: str) -> Optional[Dict[str, Any]]:
         """
         Get error information for a plugin.
@@ -287,10 +292,12 @@ class PluginStateManager:
         """Record that plugin update() was called."""
         self._last_update[plugin_id] = datetime.now()
     
+    @deprecated("3.10.0")
     def get_last_update(self, plugin_id: str) -> Optional[datetime]:
         """Get timestamp of last update() call."""
         return self._last_update.get(plugin_id)
 
+    @deprecated("3.10.0", "use get_state()")
     def get_state_info(self, plugin_id: str) -> Dict[str, Any]:
         """
         Get comprehensive state information for a plugin.