feat(profile): add user profile management with first and last name fields

backend/prisma/migrations/20260417300000_add_user_profile_fields/migration.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `User` ADD COLUMN `firstName` VARCHAR(191) NULL;
+ALTER TABLE `User` ADD COLUMN `lastName` VARCHAR(191) NULL;

backend/prisma/schema.prisma

@@ -11,6 +11,8 @@ model User {
   id           Int          @id @default(autoincrement())
   username     String       @unique
   email        String       @unique
+  firstName    String?
+  lastName     String?
   passwordHash String
   createdAt    DateTime     @default(now())
   updatedAt    DateTime     @updatedAt