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_vegas_density.py

@@ -3,6 +3,7 @@ Tests for the Vegas mode density work: dead-space trimming in PluginAdapter
 and the configurable lead-in gap in ScrollHelper.
 """
 
+import dataclasses
 from contextlib import contextmanager
 
 import pytest
@@ -298,9 +299,6 @@ class TestPluginBoundaryGaps:
             def get_grouped_content_for_composition(self):
                 return grouped
 
-            def get_active_plugin_ids(self):
-                return [pid for pid, _ in grouped]
-
         class DM:
             width = DISPLAY_W
             height = DISPLAY_H
@@ -615,10 +613,10 @@ class TestConfigSurface:
         assert cfg.lead_in_width == 0
         assert cfg.content_padding == 8
 
-    def test_round_trips_through_to_dict(self):
+    def test_round_trips_through_asdict(self):
         cfg = VegasModeConfig(trim_threshold=20, lead_in_width=64)
         restored = VegasModeConfig.from_config(
-            {'display': {'vegas_scroll': cfg.to_dict()}})
+            {'display': {'vegas_scroll': dataclasses.asdict(cfg)}})
         assert restored.trim_threshold == 20
         assert restored.lead_in_width == 64
 
@@ -698,9 +696,6 @@ class TestMeasuredSeparation:
             def get_grouped_content_for_composition(self):
                 return grouped
 
-            def get_active_plugin_ids(self):
-                return [pid for pid, _ in grouped]
-
         class DM:
             width = DISPLAY_W
             height = DISPLAY_H
@@ -816,9 +811,6 @@ class TestCycleEndsBeforeWrap:
             def get_grouped_content_for_composition(self):
                 return [('a', [Image.new('RGB', (strip_width, DISPLAY_H), (255, 255, 255))])]
 
-            def get_active_plugin_ids(self):
-                return ['a']
-
         class DM:
             width = DISPLAY_W
             height = DISPLAY_H
@@ -962,9 +954,6 @@ class TestBudgetUsesMeasuredGaps:
             def get_grouped_content_for_composition(self):
                 return [('rows', selected)]
 
-            def get_active_plugin_ids(self):
-                return ['rows']
-
         class DM:
             width = DISPLAY_W
             height = DISPLAY_H
@@ -1258,9 +1247,6 @@ class TestContinuousExtension:
             def get_grouped_content_for_composition(self):
                 return groups[0] if groups else []
 
-            def get_active_plugin_ids(self):
-                return [pid for pid, _ in (groups[0] if groups else [])]
-
             def take_next_group(self, count=None, offscreen_only=False):
                 self.calls.append(offscreen_only)
                 if self._i >= len(groups):
@@ -1450,9 +1436,6 @@ class TestDeferredDraining:
             def get_grouped_content_for_composition(self):
                 return [('seed', [Image.new('RGB', (600, DISPLAY_H), (255, 255, 255))])]
 
-            def get_active_plugin_ids(self):
-                return ['seed']
-
             def take_next_group(self, count=None, offscreen_only=False):
                 if self._served:
                     return []