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/ipc/contract.py

@@ -399,9 +399,6 @@ class HelloArgs:
     versions: Tuple[int, ...] = (PROTOCOL_VERSION,)
     client: str = ''
 
-    def to_dict(self) -> Dict[str, Any]:
-        return {'versions': list(self.versions), 'client': self.client}
-
     @classmethod
     def from_dict(cls, args: Mapping[str, Any]) -> 'HelloArgs':
         versions = args.get('versions', [PROTOCOL_VERSION])
@@ -426,10 +423,6 @@ class OnDemandStartArgs:
     duration: Optional[float] = None
     pinned: bool = False
 
-    def to_dict(self) -> Dict[str, Any]:
-        return {'plugin_id': self.plugin_id, 'mode': self.mode,
-                'duration': self.duration, 'pinned': self.pinned}
-
     @classmethod
     def from_dict(cls, args: Mapping[str, Any]) -> 'OnDemandStartArgs':
         plugin_id = _optional_name(args, 'plugin_id')
@@ -449,9 +442,6 @@ class OnDemandStartArgs:
 class OnDemandStopArgs:
     """``on_demand.stop``: end the on-demand session and resume rotation."""
 
-    def to_dict(self) -> Dict[str, Any]:
-        return {}
-
     @classmethod
     def from_dict(cls, args: Mapping[str, Any]) -> 'OnDemandStopArgs':
         return cls()
@@ -461,9 +451,6 @@ class OnDemandStopArgs:
 class NoArgs:
     """``ping`` and ``on_demand.status`` take no arguments (extra ones are ignored)."""
 
-    def to_dict(self) -> Dict[str, Any]:
-        return {}
-
     @classmethod
     def from_dict(cls, args: Mapping[str, Any]) -> 'NoArgs':
         return cls()
@@ -480,9 +467,6 @@ class BrightnessSetArgs:
     """
     brightness: int
 
-    def to_dict(self) -> Dict[str, Any]:
-        return {'brightness': self.brightness}
-
     @classmethod
     def from_dict(cls, args: Mapping[str, Any]) -> 'BrightnessSetArgs':
         value = args.get('brightness')
@@ -503,9 +487,6 @@ class PluginReloadArgs:
     """
     plugin_id: str
 
-    def to_dict(self) -> Dict[str, Any]:
-        return {'plugin_id': self.plugin_id}
-
     @classmethod
     def from_dict(cls, args: Mapping[str, Any]) -> 'PluginReloadArgs':
         plugin_id = _optional_name(args, 'plugin_id')
@@ -546,9 +527,6 @@ class StateGetArgs:
     since: Optional[int] = None
     epoch: Optional[str] = None
 
-    def to_dict(self) -> Dict[str, Any]:
-        return {'since': self.since, 'epoch': self.epoch}
-
     @classmethod
     def from_dict(cls, args: Mapping[str, Any]) -> 'StateGetArgs':
         return cls(since=_optional_version(args, 'since'), epoch=_optional_epoch(args))
@@ -565,9 +543,6 @@ class StateSubscribeArgs:
     and a ``tick`` at least every :data:`SUBSCRIBE_KEEPALIVE_SECONDS`.
     """
 
-    def to_dict(self) -> Dict[str, Any]:
-        return {}
-
     @classmethod
     def from_dict(cls, args: Mapping[str, Any]) -> 'StateSubscribeArgs':
         return cls()
@@ -582,9 +557,6 @@ class ErrorsClearArgs:
     """
     cutoff: float
 
-    def to_dict(self) -> Dict[str, Any]:
-        return {'cutoff': self.cutoff}
-
     @classmethod
     def from_dict(cls, args: Mapping[str, Any]) -> 'ErrorsClearArgs':
         value = args.get('cutoff')