feat(ST-507): add optional email field to User schema

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

prisma/migrations/20260426193458_add_user_email/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable
+ALTER TABLE "users" ADD COLUMN "email" TEXT;
+
+-- CreateIndex
+CREATE UNIQUE INDEX "users_email_key" ON "users"("email");

prisma/schema.prisma

@@ -49,6 +49,7 @@ enum SprintStatus {
 model User {
   id               String          @id @default(cuid())
   username         String          @unique
+  email            String?         @unique
   password_hash    String
   is_demo          Boolean         @default(false)
   bio              String?         @db.VarChar(160)