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/font_manager.py

@@ -41,6 +41,7 @@ from PIL import ImageFont
 from src.common.bdf_font import load_bdf_face, read_bdf_native_size
 from src.common.font_layout import load_truetype, resolve_asset_path
 from typing import Dict, Tuple, Optional, Union, Any
+from src.deprecation import deprecated
 
 logger = logging.getLogger(__name__)
 
@@ -533,6 +534,7 @@ class FontManager:
         """
         return load_bdf_face(font_path, size_px)[0]
 
+    @deprecated("3.10.0", "use src.common.bdf_font.read_bdf_native_size()")
     def get_native_bdf_size(self, family: str) -> Optional[int]:
         """The one true pixel size of a BDF family in the catalog, or None
         for scalable (TTF) families / unknown families."""
@@ -553,6 +555,7 @@ class FontManager:
 
     # ==================== Font Measurement ====================
 
+    @deprecated("3.10.0", "use src.adaptive_layout.measure_ink()")
     def measure_text(self, text: str, font: Union[ImageFont.FreeTypeFont, freetype.Face]) -> Tuple[int, int, int]:
         """
         Measure text dimensions and baseline.