Improve family invitations and absence management flows

prisma/schema.prisma

@@ -265,6 +265,7 @@ model User {
 
   mitbringselItems   MitbringselItem[]
   invitationsCreated Invitation[]                 @relation("InvitationCreator")
+  familyInvitation   Invitation?                  @relation("InvitationRecipient")
   auditLogsAuthored  AuditLog[]                   @relation("AuditLogActor")
   staffProfile       StaffProfile?
   staffNotifications StaffNotification[]
@@ -737,10 +738,19 @@ model Invitation {
   email String
   role  UserRole @default(ELTERN)
 
+  // Nullable for older, unlinked invitations. Family invitations belong to a user.
+  userId String? @unique
+  user   User?   @relation("InvitationRecipient", fields: [userId], references: [id], onDelete: Cascade)
+
   token      String           @unique
   status     InvitationStatus @default(PENDING)
   expiresAt  DateTime
   acceptedAt DateTime?
+  sentAt        DateTime?
+  lastAttemptAt DateTime?
+  leaseUntil    DateTime?
+  attempts      Int       @default(0)
+  failed        Boolean   @default(false)
 
   invitedById String?
   invitedBy   User?   @relation("InvitationCreator", fields: [invitedById], references: [id], onDelete: SetNull)