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/base_odds_manager.py

@@ -21,6 +21,7 @@ from typing import Dict, Any, Optional, List, cast
 from src.common.api_helper import DEFAULT_HTTP_HEADERS
 from src.common.fetch_service import fetch_get, share_connection_pool
 from src.common.json_body import response_json
+from src.deprecation import deprecated
 
 
 
@@ -277,6 +278,7 @@ class BaseOddsManager:
             self.logger.warning(f"Unexpected response structure: {json.dumps(data, indent=2)}")
             return None
     
+    @deprecated("3.10.0", "call get_odds() for each game")
     def get_odds_for_games(self, games: List[Dict[str, Any]]) -> List[Dict[str, Any]]:
         """
         Fetch odds for multiple games efficiently.
@@ -335,6 +337,7 @@ class BaseOddsManager:
             
         return False
     
+    @deprecated("3.10.0")
     def format_odds_summary(self, odds_data: Optional[Dict[str, Any]]) -> str:
         """
         Format odds data into a human-readable summary.