Phase 2: Parallelize frontend validation from backend test execution

[dgee2]

Summary

Restructure the workflow so frontend validation no longer waits behind the full backend lane.

Change

Create separate jobs for:

• backend tests
• frontend validation
• optional small OpenAPI-producing artifact job if required by the frontend

Why

The main structural bottleneck is serialization. Frontend work currently trails the backend instead of running concurrently.

Acceptance criteria

• Frontend runs in a separate job
• Frontend depends only on the minimum backend artifact it needs
• Total wall-clock time improves without reducing coverage
• Workflow graph remains clear and maintainable