fix: use upsert to prevent race condition in addOrganizationMember

Concurrent invites for the same user could both pass the existence check
and then the second create() would throw a unique constraint violation.

js/cf-webapp/src/app/(dashboard)/members/action.ts

@@ -131,19 +131,16 @@ export async function addOrganizationMember(
       return createErrorResponse("User is already a member of this organization")
     }
 
-    // Check if user exists in our database
-    let user = await getUserById(invitedUserId)
-
-    // If user doesn't exist, create them
-    if (!user) {
-      user = await prisma.users.create({
-        data: {
-          user_id: invitedUserId,
-          github_username: invitedUser.username,
-          onboarding_completed: false,
-        },
-      })
-    }
+    // Ensure user exists in our database (upsert handles concurrent invites safely)
+    const user = await prisma.users.upsert({
+      where: { user_id: invitedUserId },
+      update: {},
+      create: {
+        user_id: invitedUserId,
+        github_username: invitedUser.username,
+        onboarding_completed: false,
+      },
+    })
 
     // Add user to organization members
     const newMember = await prisma.organization_members.create({