Backend: Delete BTC certificiranje

GitHub Issue: #254

Backend task for deleting a BTC certificiranje (DELETE /btc-certifications/{certificationId}) – soft delete, check permissions and the upcoming-only rule, implement, and verify against the spec.

Docs: BTC certificiranje list (Actions menu → Ukloni certificiranje, Delete confirmation modal) Spec: DELETE /btc-certifications/{certificationId} (deleteBtcCertification), BtcCertificationConflict in api-docs/openapi.yaml DB: BtcCertifications (DeletedAt) in api/database/create_schema.sql

1. Double check API spec and DB structure

• Soft delete (decided): the spec soft-deletes (sets DeletedAt) and keeps candidates, komisija and grades. Data is kept for 2 years (see BTC), the same as Backend: Delete BTC školovanje and Assessments.
• Rule: only upcoming can be deleted → 409 certification_not_upcoming. Check the docs agree (Aktivni and Prošli can’t be deleted).
• After deleting, every BTC certificiranje endpoint (detail, edit, oral exam form, results, written exam scores) → 404, and the list excludes it.
• Roles and permissions: btc-certifications.edit, Trainer only (BTC evaluators can’t delete; see Roles).

2. Implementation

• Roles and permissions: require btc-certifications.edit → 403.
• DB: set DeletedAt (and UpdatedAt). Make sure the shared “load certificiranje” helper used by the other endpoints filters DeletedAt IS NULL.
• Status check with the shared status helper → 409. Unknown or already deleted id → 404. Success → 204.

3. Verify implementation against spec

• Status codes and the error value match deleteBtcCertification.
• Active / past certificiranje → 409. BTC evaluator → 403. Deleting twice → 404 the second time.
• After deleting: list excludes it; detail, oral exam form and results → 404.

Done when

A trainer can delete an upcoming certificiranje, and it disappears everywhere in the app while its data stays in the database.