feat(auth): default to single-user mode with enable toggle

backend/prisma/migrations/20260206173000_add_auth_enabled_system_config/migration.sql

@@ -0,0 +1,5 @@
+-- Add authEnabled flag to SystemConfig to support single-user mode by default.
+
+-- SQLite supports simple ADD COLUMN for non-null with default.
+ALTER TABLE "SystemConfig" ADD COLUMN "authEnabled" BOOLEAN NOT NULL DEFAULT false;
+

backend/prisma/schema.prisma

@@ -32,6 +32,7 @@ model User {
 
 model SystemConfig {
   id                  String   @id @default("default")
+  authEnabled         Boolean  @default(false)
   registrationEnabled Boolean  @default(false)
   createdAt           DateTime @default(now())
   updatedAt           DateTime @updatedAt