feat: username 90-day change restriction

prisma/migrations/20250418153435_username_updated_at/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "users" ADD COLUMN     "usernameUpdatedAt" TIMESTAMP(3);

prisma/migrations/migration_lock.toml

@@ -1,3 +1,3 @@
 # Please do not edit this file manually
 # It should be added in your version-control system (e.g., Git)
-provider = "postgresql"
+provider = "postgresql"

prisma/schema.prisma

@@ -18,6 +18,8 @@ model User {
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
 
+  usernameUpdatedAt DateTime?
+
   accounts Account[]
   sessions Session[]
   miis     Mii[]