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/tests/storage/test_base_store.py

@@ -1,7 +1,6 @@
 """Tests for BaseJsonStore — the shared data-layer base class."""
 
 import json
-import threading
 from concurrent.futures import ThreadPoolExecutor, as_completed
 from dataclasses import dataclass
 from pathlib import Path

server/tests/storage/test_device_store.py

@@ -1,6 +1,5 @@
 """Tests for DeviceStore — device CRUD, persistence, name uniqueness, thread safety."""
 
-import threading
 from concurrent.futures import ThreadPoolExecutor, as_completed
 from pathlib import Path
 

server/tests/storage/test_output_target_store.py

@@ -7,7 +7,6 @@ from wled_controller.storage.output_target_store import OutputTargetStore
 from wled_controller.storage.wled_output_target import WledOutputTarget
 from wled_controller.storage.key_colors_output_target import (
     KeyColorsOutputTarget,
-    KeyColorsSettings,
 )