feature: localization's first implmentation

backend/prisma/migrations/20260620120000_add_user_language/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "users" ADD COLUMN "language" TEXT NOT NULL DEFAULT 'en';

backend/prisma/schema.prisma

@@ -15,6 +15,7 @@ model User {
   googleId     String? @unique
   facebookId   String? @unique
   name         String
+  language     String  @default("en")
   trialUsedAt  DateTime?
 
   memberships        Membership[]