feat: add trainee self reflection API (T-313)

[xfoxfu]

Summary

Add configurable trainee self-reflection to training records. Seed a training-self-reflection sheet with one free-text field, expose its active fields, and return submitted answers with the existing training DTO.

The trainee or a user with controller-training-director-assistant permission can save or edit the reflection at any time. Reading follows existing training-record permissions, including other mentors. The sheet endpoint is scoped to the training ID and uses the same visibility check; training DTOs return reflection answers without additional filtering. Existing role inheritance applies. Saves reuse the sheet filing in a transaction with a training-row lock, including concurrent first submissions.

Validation

• 8 API tests passed, covering field configuration, create/edit/read, access control, concurrent submissions, invalid-answer rollback, and existing training endpoints.
• Compiled backend and generated OpenAPI types.
• Applied the new migration successfully to an isolated PostgreSQL database.

Deployment

Apply 20260921000000_add_training_self_reflection.sql before deploying the API and the companion frontend change. Staff can configure the sheet through the existing sheet management system.

Implements T-313. Mentor feedback revision history (T-314) is outside this change.

Companion frontend: VATPRChina/vatprc-docs#84.

[linear-code]

T-313