feature: sending invitation link for newly created staff implemented.

backend/prisma/schema.prisma

@@ -20,6 +20,7 @@ model User {
   memberships        Membership[]
   ownedOrganizations Organization[] @relation("OrganizationOwner")
   sessions           Session[] // 👈 ADD THIS - opposite relation for Session
+  sentStaffInvites   StaffInvitation[]
 
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
@@ -122,11 +123,13 @@ model Membership {
   organizationId String
 
   isOwner Boolean @default(false)
+  isActive Boolean @default(true)
 
   user         User         @relation(fields: [userId], references: [id])
   organization Organization @relation(fields: [organizationId], references: [id])
 
   permissions MembershipPermission[]
+  invitations StaffInvitation[]
 
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
@@ -135,6 +138,26 @@ model Membership {
   @@map("memberships")
 }
 
+model StaffInvitation {
+  id String @id @default(uuid())
+
+  membershipId String
+  invitedById  String
+  tokenHash    String   @unique
+  expiresAt    DateTime
+  acceptedAt   DateTime?
+  revokedAt    DateTime?
+
+  membership Membership @relation(fields: [membershipId], references: [id], onDelete: Cascade)
+  invitedBy  User       @relation(fields: [invitedById], references: [id], onDelete: Cascade)
+
+  createdAt DateTime @default(now())
+  updatedAt DateTime @updatedAt
+
+  @@index([membershipId, createdAt])
+  @@map("staff_invitations")
+}
+
 model Permission {
   id          String  @id @default(uuid())
   name        String  @unique