Backend: Delete BTC školovanje

GitHub Issue: #247

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

Docs: BTC školovanje list (Actions menu → Ukloni školovanje, Delete confirmation modal) Spec: DELETE /btc-trainings/{trainingId} (deleteBtcTraining), BtcTrainingConflict in api-docs/openapi.yaml DB: BtcTrainings (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, attendance, feedback values and signature files. Data is kept for 2 years (see BTC). Assessments and BTC certificiranje use the same approach.
• Rule: only upcoming can be deleted → 409 training_not_upcoming. Check the docs agree (Aktivni and Prošli can’t be deleted).
• After deleting, every BTC školovanje endpoint (detail, edit, attendance, feedback, signature files, download) → 404, and the list excludes it.
• Roles and permissions: btc-trainings.edit, Trainer only.

2. Implementation

• Roles and permissions: require btc-trainings.edit → 403.
• DB: set DeletedAt (and UpdatedAt). Make sure the shared “load školovanje” 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 deleteBtcTraining.
• Active / past školovanje → 409. Evaluator → 403. Deleting twice → 404 the second time.
• After deleting: list excludes it, detail and signature download → 404.

Done when

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