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/api/routes/test_devices_routes.py

@@ -4,8 +4,7 @@ These tests exercise the FastAPI route handlers using dependency override
 to inject test stores, avoiding real hardware dependencies.
 """
 
-from datetime import datetime, timezone
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 from fastapi import FastAPI