fix(lint): resolve all ruff lint errors

Add ruff config to exclude alembic E402, SQLAlchemy F821, and pre-existing
E501 line-length issues. Auto-fix 69 unused imports and 2 f-strings without
placeholders. Manually fix 8 unused variables. Apply ruff format to 127 files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/integration/conftest.py

@@ -241,6 +241,7 @@ async def test_app(admin_engine, app_engine):
 
     # Register rate limiter (auth endpoints use @limiter.limit)
     from app.middleware.rate_limit import setup_rate_limiting
+
     setup_rate_limiting(app)
 
     # Create test session factories