Wrap first-user admin check in transaction to prevent race condition

Co-authored-by: xCyanGrizzly <53275238+xCyanGrizzly@users.noreply.github.com>

src/app/(auth)/register/actions.ts

@@ -22,10 +22,11 @@ export async function registerUser(input: unknown): Promise<ActionResult<{ id: s
   const hashedPassword = await bcrypt.hash(parsed.data.password, 10);
 
   // First user to register becomes ADMIN (self-hosted owner)
-  const userCount = await prisma.user.count();
+  const user = await prisma.$transaction(async (tx) => {
+    const userCount = await tx.user.count();
     const role = userCount === 0 ? "ADMIN" : "USER";
 
-  const user = await prisma.user.create({
+    return tx.user.create({
       data: {
         name: parsed.data.name,
         email: parsed.data.email,
@@ -41,6 +42,7 @@ export async function registerUser(input: unknown): Promise<ActionResult<{ id: s
         },
       },
     });
+  });
 
   return { success: true, data: { id: user.id } };
 }