feat: Admin API to mark mentor unavailable and forward mentee to next priority

[womencodingcommunity]

Context

Two scenarios require forwarding a mentee to their next priority mentor without the mentor actively declining:

1. Mentor unavailable: mentor cannot participate this cycle (capacity full, on leave, etc.) — unrelated to the specific mentee
2. Admin forwarded: mentor has not responded within the expected timeframe and admin manually escalates

Both statuses trigger the same forwarding logic but are semantically distinct for reporting purposes.

Changes Required

MenteeWorkflowService

• Add markMentorUnavailable(applicationId, reason):
  • Sets status to MENTOR_UNAVAILABLE
  • Triggers next priority forwarding
• Add forwardApplication(applicationId, reason):
  • Sets status to ADMIN_FORWARDED
  • Triggers next priority forwarding
• Both methods call checkAndTriggerManualMatch() if no more priorities remain

New API endpoints

PATCH /api/platform/v1/mentors/applications/{applicationId}/mark-unavailable
Body: { "reason": "string" }

PATCH /api/platform/v1/mentees/applications/{applicationId}/forward
Body: { "reason": "string" }

Acceptance Criteria

• markMentorUnavailable() implemented and tested
• forwardApplication() implemented and tested
• Both trigger next priority forwarding
• Both trigger PENDING_MANUAL_MATCH when priorities exhausted
• Both endpoints documented in OpenAPI/Swagger
• Unit and integration tests

Dependencies

• Ticket: feat: Add new ApplicationStatus values and DB migration
• Ticket: feat: PENDING_MANUAL_MATCH detection and manual matching queue