ci: enable ruff linting and pytest in CI pipeline

Uncomment the ruff check and pytest steps in the Gitea Actions build
workflow so that linting violations and test failures block image builds.
Fix all pre-existing ruff violations (E402 import ordering in analyzer.py,
F821 undefined name in api.py, I001 unsorted imports in test files, F401
unused import in test_rate_limit.py).

Closes leeworks-agents/SPARC#1559
Closes leeworks-agents/SPARC#1560

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_rate_limit.py

@@ -1,7 +1,8 @@
 """Tests for rate limiting on auth endpoints."""
 
+from unittest.mock import MagicMock, patch
+
 import pytest
-from unittest.mock import Mock, patch, MagicMock
 from fastapi.testclient import TestClient
 
 from SPARC.api import app