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/test_processor_manager.py

@@ -1,9 +1,6 @@
 """Tests for processor manager."""
 
-import asyncio
 import pytest
-import respx
-from httpx import Response
 
 from wled_controller.core.processing.processor_manager import ProcessorManager
 from wled_controller.core.processing.processing_settings import ProcessingSettings