IAV-1155 : add token status endpoint, fix N+1, persist expiry, centralize constants

• Extract InvitationTokenPair to dtos package with proper encapsulation
• Add invitationTokens list to CollaboratorDto
• Add UserInvitationTokenController GET /api/user/invitation-tokens/verify
• Add getTokenStatus() in service to centralize token status logic
• Fix PENDING->EXPIRED not persisted via @Modifying JPQL UPDATE (REQUIRES_NEW)
• Fix N+1: replace invalidateAllPending loop with single bulk UPDATE query
• Extend token validity from 48h to 72h
• Add EmailConstants class for invitation template and subject
• Translate invitation email subject to French

Closes IAV-1155