Split backend unit and integration test suites

[nensii21]

Summary

• Split backend test execution into separate unit and integration jobs
• Preserved benchmark exclusion with -m "not benchmark"
• Improved CI failure visibility by reporting unit and integration failures independently

Validation

• Unit test suite runs independently
• Integration test suite runs independently
• Benchmark exclusion behavior unchanged

[nensii21]

The workflow has been updated to split backend testing into separate unit and integration jobs.

Current CI failures appear unrelated to this change:

• backend-lint fails due to an existing Ruff error (F821 Undefined name 'db' in backend/secuscan/workflows.py)
• frontend-checks fails in an existing frontend test (testing/unit/pages/Workflows.test.tsx)

The new backend-unit-tests and backend-integration-tests jobs are present in the workflow but are skipped because they depend on backend-lint succeeding.

GitHub is also still expecting the legacy backend-tests status check, which appears to come from branch protection settings.