fix: resolve all 153 ruff lint errors for CI

Auto-fixed 138 unused imports and f-string issues. Manually fixed:
ambiguous variable names (l→layer), availability-check imports using
importlib.util.find_spec, unused Color import, ImagePool forward ref
via TYPE_CHECKING, multi-statement semicolons, and E402 suppression.

server/src/wled_controller/api/routes/audio_templates.py

@@ -1,7 +1,6 @@
 """Audio capture template and engine routes."""
 
 import asyncio
-import json
 from fastapi import APIRouter, HTTPException, Depends, Query
 from starlette.websockets import WebSocket, WebSocketDisconnect