472 Release Track Endpoint Updates

[clemiller]

Updates release track responses with UI-friendly summary and object details:

• Adds summary counts to GET /api/release-tracks for each track:
  • members_count
  • staged_count
  • candidates_count
• Adds object details to candidates and staged entries in GET /api/release-tracks/:id:
  • attack_id
  • name
  • description
  • modified_by_user