improvement: multi organization possibility implemented for users (owners and staffs)

backend/prisma/migrations/20260429134000_add_user_trial_used_at/migration.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "users"
+ADD COLUMN "trialUsedAt" TIMESTAMP(3);

backend/prisma/schema.prisma

@@ -15,6 +15,7 @@ model User {
   googleId     String? @unique
   facebookId   String? @unique
   name         String
+  trialUsedAt  DateTime?
 
   memberships        Membership[]
   ownedOrganizations Organization[] @relation("OrganizationOwner")