diff --git a/apps/backend/prisma/migrations/20260323000000_add_signed_up_at_default/migration.sql b/apps/backend/prisma/migrations/20260323000000_add_signed_up_at_default/migration.sql
new file mode 100644
index 0000000000..f404be36f3
--- /dev/null
+++ b/apps/backend/prisma/migrations/20260323000000_add_signed_up_at_default/migration.sql
@@ -0,0 +1,8 @@
+-- Backward-compat: old code that doesn't know about `signedUpAt` omits it from
+-- INSERT.  Adding a DEFAULT lets Postgres fill it automatically.
+--
+-- CURRENT_TIMESTAMP is correct here: `createdAt` also defaults to
+-- CURRENT_TIMESTAMP, so within the same transaction both columns receive the
+-- same value.  Old code never computes risk scores, so the negligible edge
+-- case of an explicitly-backdated `createdAt` is harmless.
+ALTER TABLE "ProjectUser" ALTER COLUMN "signedUpAt" SET DEFAULT CURRENT_TIMESTAMP;
diff --git a/apps/backend/prisma/schema.prisma b/apps/backend/prisma/schema.prisma
index ef7b4b6ae9..ecb4a8c512 100644
--- a/apps/backend/prisma/schema.prisma
+++ b/apps/backend/prisma/schema.prisma
@@ -276,7 +276,7 @@ model ProjectUser {
   restrictedByAdminPrivateDetails String? // Private details (server access only)
 
   // Sign-up metadata
-  signedUpAt                      DateTime
+  signedUpAt                      DateTime @default(now())
   signUpIp                        String?
   signUpIpTrusted                 Boolean?
   signUpEmailNormalized           String?