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_auth.py

@@ -175,6 +175,7 @@ class TestGetMe:
         from datetime import timedelta
 
         import jwt as pyjwt
+
         from SPARC.auth import JWT_ALGORITHM, JWT_SECRET
 
         payload = {