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/dynamic_team_resolver.py

@@ -24,6 +24,7 @@ from typing import Any, Dict, List
 
 from src.common.api_helper import DEFAULT_HTTP_HEADERS
 from src.common.json_body import response_json
+from src.deprecation import deprecated
 
 logger = logging.getLogger(__name__)
 
@@ -201,6 +202,7 @@ class DynamicTeamResolver:
         DynamicTeamResolver._failure_timestamp = current_time
         return {}
     
+    @deprecated("3.10.0", "use resolve_teams()")
     def get_available_dynamic_teams(self) -> List[str]:
         """
         Get list of available dynamic team names.
@@ -210,6 +212,7 @@ class DynamicTeamResolver:
         """
         return list(self.DYNAMIC_PATTERNS.keys())
     
+    @deprecated("3.10.0", "use resolve_teams()")
     def is_dynamic_team(self, team_name: str) -> bool:
         """
         Check if a team name is a dynamic team.