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/schemas/common.py

@@ -1,7 +1,7 @@
 """Shared schemas used across multiple route modules."""
 
 from datetime import datetime
-from typing import Dict, List, Optional
+from typing import Dict, Optional
 
 from pydantic import BaseModel, Field