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/test_alerts_api.py

@@ -258,9 +258,7 @@ class TestAlertEvents:
     ):
         """GET /api/tenants/{tenant_id}/devices/{device_id}/alerts returns paginated response."""
         tenant = await create_test_tenant(admin_session)
-        auth = await auth_headers_factory(
-            admin_session, existing_tenant_id=tenant.id
-        )
+        auth = await auth_headers_factory(admin_session, existing_tenant_id=tenant.id)
         tenant_id = auth["tenant_id"]
         device = await create_test_device(admin_session, tenant.id)
         await admin_session.commit()