feat: wire remaining dashboard placeholder routes to API

[phibkro]

Summary

5 dashboard routes still show placeholder headings because they lack corresponding API endpoints:

• dashboard.attester._index.tsx — Certificates
• dashboard.avdelinger._index.tsx — Departments (admin CRUD)
• dashboard.intervjusjema._index.tsx — Interview schemas
• dashboard.opptaksperioder._index.tsx — Admission periods
• dashboard.intervjufordeling._index.tsx — Interview distribution
• dashboard.tidligere-assistenter._index.tsx — Former assistants

Tasks

1. Add missing list endpoints to Symfony API
2. Wire dashboard loaders to new endpoints
3. Add data tables matching existing pattern (see dashboard.sokere._index.tsx)