emailにUnique制約を追加

prisma/migrations/20240616122115_make_email_unique/migration.sql

@@ -0,0 +1,8 @@
+/*
+  Warnings:
+
+  - A unique constraint covering the columns `[email]` on the table `User` will be added. If there are existing duplicate values, this will fail.
+
+*/
+-- CreateIndex
+CREATE UNIQUE INDEX "User_email_key" ON "User"("email");

prisma/schema.prisma

@@ -15,7 +15,7 @@ datasource db {
 
 model User {
     id String @id @default(cuid())
-    email String
+    email String @unique
     password String
     name String
     created_at DateTime @default(now())