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/cache/disk_cache.py

@@ -15,11 +15,14 @@ import tempfile
 import logging
 import threading
 import zlib
-from typing import Dict, Any, Optional, Protocol, Tuple
+from typing import TYPE_CHECKING, Dict, Any, Optional, Tuple
 from datetime import datetime
 
 from src.common.path_safety import safe_path_component
 
+if TYPE_CHECKING:
+    from src.cache.cache_strategy import CacheStrategy
+
 try:  # optional: large speedup on the cache write path, see _dumps below
     import orjson
 except ImportError:  # pragma: no cover - exercised on hosts without the wheel
@@ -62,23 +65,6 @@ def _filename_stem(key: str) -> str:
     return f"{prefix}-{digest}"
 
 
-
-class CacheStrategyProtocol(Protocol):
-    """Protocol for cache strategy objects that categorize cache keys."""
-    
-    def get_data_type_from_key(self, key: str) -> str:
-        """
-        Determine the data type from a cache key.
-        
-        Args:
-            key: Cache key
-            
-        Returns:
-            Data type string for strategy lookup
-        """
-        ...
-
-
 class DateTimeEncoder(json.JSONEncoder):
     """JSON encoder that handles datetime objects.
 
@@ -816,12 +802,12 @@ class DiskCache:
         # mkstemp's random component.
         return bool(sep) and len(head) > 1 and bool(suffix)
 
-    def cleanup_expired_files(self, cache_strategy: CacheStrategyProtocol, retention_policies: Dict[str, int]) -> Dict[str, Any]:
+    def cleanup_expired_files(self, cache_strategy: 'CacheStrategy', retention_policies: Dict[str, int]) -> Dict[str, Any]:
         """
         Clean up expired cache files based on retention policies.
         
         Args:
-            cache_strategy: Object implementing CacheStrategyProtocol for categorizing files
+            cache_strategy: Categorizes files by key (get_data_type_from_key)
             retention_policies: Dict mapping data types to retention days
             
         Returns: