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/_preview_helpers.py

@@ -3,16 +3,14 @@
 import asyncio
 import base64
 import io
-import secrets
 import threading
 import time
-from typing import Callable, List, Optional
+from typing import Callable, Optional
 
 import numpy as np
 from PIL import Image
 from starlette.websockets import WebSocket
 
-from wled_controller.config import get_config
 from wled_controller.core.filters import FilterRegistry, ImagePool
 from wled_controller.utils import get_logger