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/test_error_aggregator.py

@@ -218,24 +218,6 @@ class TestPatternDetection:
         assert pattern is not None
         assert pattern.severity in ["error", "critical"]
 
-    def test_pattern_callback_called(self):
-        """Pattern detection callback should be called."""
-        aggregator = ErrorAggregator(pattern_threshold=2)
-
-        callback_called = []
-
-        def callback(pattern):
-            callback_called.append(pattern)
-
-        aggregator.on_pattern_detected(callback)
-
-        # Trigger pattern
-        for _ in range(3):
-            aggregator.record_error(error=ValueError("Pattern trigger"))
-
-        assert len(callback_called) == 1
-        assert callback_called[0].error_type == "ValueError"
-
 
 class TestErrorSummary:
     """Test error summary generation."""
@@ -318,37 +300,6 @@ class TestPluginHealth:
         assert health["recent_error_count"] == 10
 
 
-class TestRecordClearing:
-    """Test clearing old records."""
-
-    def test_clear_old_records(self):
-        """Old records should be cleared."""
-        aggregator = ErrorAggregator()
-
-        # Add a record
-        aggregator.record_error(error=ValueError("Old error"))
-
-        # Manually age the record
-        aggregator._records[0].timestamp = datetime.now() - timedelta(hours=48)
-
-        # Clear records older than 24 hours
-        cleared = aggregator.clear_old_records(max_age_hours=24)
-
-        assert cleared == 1
-        assert len(aggregator._records) == 0
-
-    def test_recent_records_not_cleared(self):
-        """Recent records should not be cleared."""
-        aggregator = ErrorAggregator()
-
-        aggregator.record_error(error=ValueError("Recent error"))
-
-        cleared = aggregator.clear_old_records(max_age_hours=24)
-
-        assert cleared == 0
-        assert len(aggregator._records) == 1
-
-
 class TestThreadSafety:
     """Test thread safety of error aggregator."""