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>

scripts/dev/README.md

@@ -5,7 +5,6 @@ This directory contains scripts and utilities for development and testing.
 ## Scripts
 
 - **`dev_plugin_setup.sh`** - Sets up plugin development environment by linking plugin repositories
-- **`run_emulator.sh`** - Runs the LED Matrix display in emulator mode (for development without hardware)
 - **`vegas_audit.py`** - Measures how much of the Vegas ticker strip actually shows content (dead-frame ratio)
 - **`test_pillow_compat.py`** - Pillow API smoke test to run after upgrading Pillow (`python3 scripts/dev/test_pillow_compat.py`)
 
@@ -28,6 +27,6 @@ links. To use a fork or another clone location, copy
 
 ### Running Emulator
 ```bash
-./scripts/dev/run_emulator.sh
+python3 run.py -e
 ```