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/background_data_service.py

@@ -42,6 +42,7 @@ from src.common.espn_dates import (
     fetch_espn_date_chunks,
     parse_espn_date_range,
 )
+from src.deprecation import deprecated
 # Configure logging
 logger = logging.getLogger(__name__)
 
@@ -679,6 +680,7 @@ class BackgroundDataService:
         
         raise last_exception
     
+    @deprecated("3.10.0", "pass callback= to submit_fetch_request()")
     def get_result(self, request_id: str) -> Optional[FetchResult]:
         """
         Get the result of a fetch request.
@@ -695,6 +697,7 @@ class BackgroundDataService:
         with self._lock:
             return self.completed_requests.get(request_id)
     
+    @deprecated("3.10.0", "pass callback= to submit_fetch_request()")
     def is_request_complete(self, request_id: str) -> bool:
         """
         Check if a request has completed.
@@ -711,6 +714,7 @@ class BackgroundDataService:
         with self._lock:
             return request_id in self.completed_requests
     
+    @deprecated("3.10.0", "pass callback= to submit_fetch_request()")
     def get_request_status(self, request_id: str) -> Optional[FetchStatus]:
         """
         Get the status of a fetch request.