[feat] Enrollment submission and validation API

[erikbayerlein]

Epic: #69 (Seleção de Temas e Finalização SIGAA)
Use Case: #64

Description

Finalize and validate the complete enrollment submission.

Acceptance Criteria

• Validates all required fields across all enrollment steps before allowing submission.
• Validates declaration of truthfulness is accepted.
• Changes enrollment status from draft to submitted.
• Returns detailed validation errors indicating which fields/steps are incomplete.
• Submitted enrollments can be edited while the period is open.
• Enrollment is locked (read-only) after the period closes.