feat: Missing POST/PUT endpoints to accept and decline mentor and mentee by mentorship team after registration

[nora-weisser]

Description

Mentor Approval Flow

1. Mentor submits registration
2. Endpoint: POST /mentors
3. Payload: complete mentor profile (bio, skills, focus areas, availability)

Expected result:

1. 201 Created
2. Mentor stored with status = PENDING

Mentorship Team reviews mentor profile

a) Approve mentor (Missing endpoint)
Endpoint: POST /mentors/{mentorId}/accept

Expected result:

1. 200 OK
2. Mentor stored with status = ACTIVE
3. Approval email notification sent to the mentor

a) Rejects mentor (Missing endpoint)

Endpoint: POST /mentors/{mentorId}/reject

Expected result:

1. 200 OK
2. Mentor stored with status = REJECTED
3. Rejection feedback stored (if provided)
4. Rejection email notification sent to the mentor

The same accept and reject endpoints are missing for mentee.