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>

test/web_interface/test_api_v3_helpers.py

@@ -22,7 +22,6 @@ from web_interface.blueprints.api_v3 import (  # noqa: E402
     _is_plugin_update_available,
     _coerce_to_bool,
     deep_merge,
-    _parse_form_value,
     _get_schema_property,
     _set_nested_value,
     _SKIP_FIELD,
@@ -116,44 +115,6 @@ class TestDeepMerge:
         assert result["keep"] is base["keep"]  # untouched subtree is shared
 
 
-class TestParseFormValue:
-    def test_boolean_strings(self):
-        assert _parse_form_value("true") is True
-        assert _parse_form_value("False") is False
-
-    def test_null_like_strings(self):
-        assert _parse_form_value("null") is None
-        assert _parse_form_value("none") is None
-        assert _parse_form_value("") is None
-
-    def test_none_passthrough(self):
-        assert _parse_form_value(None) is None
-
-    def test_numbers(self):
-        assert _parse_form_value("42") == 42
-        assert isinstance(_parse_form_value("42"), int)
-        assert _parse_form_value("3.5") == 3.5
-        assert isinstance(_parse_form_value("3.5"), float)
-
-    def test_json_array_parsed_before_numbers(self):
-        # RGB arrays like "[255, 0, 0]" must come back as lists.
-        assert _parse_form_value("[255, 0, 0]") == [255, 0, 0]
-
-    def test_json_object(self):
-        assert _parse_form_value('{"a": 1}') == {"a": 1}
-
-    def test_malformed_json_falls_back_to_string(self):
-        assert _parse_form_value("[not json") == "[not json"
-
-    def test_plain_string_returned_unstripped(self):
-        # The original value (not the stripped copy) is returned.
-        assert _parse_form_value("  hello  ") == "  hello  "
-
-    def test_non_string_passthrough(self):
-        assert _parse_form_value(7) == 7
-        assert _parse_form_value([1, 2]) == [1, 2]
-
-
 class TestGetSchemaProperty:
     SCHEMA: ClassVar[Dict[str, Any]] = {
         "properties": {