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_webhooks_routes.py

@@ -1,7 +1,6 @@
 """Tests for webhook routes — trigger, validation, rate limiting."""
 
 import time
-from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest