[feat] Theme selection API for enrollment

[erikbayerlein]

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

Description

API for candidates to select primary and secondary research themes during enrollment.

Acceptance Criteria

• Candidate can set primary and secondary theme choices for their enrollment.
• Primary and secondary themes must be different.
• Both themes must match the enrollment level (masters/doctoral).
• Theme selection can be updated while the enrollment period is open.
• API validates theme existence and level compatibility.