refactor: remove username from User model, login by email only

tests/integration/helpers/factories.ts

@@ -14,7 +14,6 @@ function nextSuffix() {
 export async function createTestUser(
   prisma: PrismaClient,
   overrides: Partial<{
-    username: string
     email: string
     name: string
     role: UserRole
@@ -25,7 +24,6 @@ export async function createTestUser(
 
   return prisma.user.create({
     data: {
-      username: overrides.username ?? `test-user-${suffix}`,
       email: overrides.email ?? `test-user-${suffix}@example.test`,
       name: overrides.name ?? "Test User",
       password: "hashed-password",