feat(auth): enhance authentication system with multi-user support and admin role management

- Implemented multi-user authentication with role-based access control.
- Added environment variables for initial admin user setup.
- Updated README and example environment file with new authentication options.
- Introduced user and system configuration models in the database schema.
- Enhanced authentication middleware to support user registration and role management.
- Updated frontend to handle new authentication flows, including admin user creation and role updates.

backend/prisma/schema.prisma

@@ -40,3 +40,20 @@ model Library {
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
 }
+
+model User {
+  id           String  @id @default(uuid())
+  username     String? @unique
+  email        String? @unique
+  passwordHash String
+  role         String  @default("USER")
+  createdAt    DateTime @default(now())
+  updatedAt    DateTime @updatedAt
+}
+
+model SystemConfig {
+  id                  String   @id @default("default")
+  registrationEnabled Boolean  @default(false)
+  createdAt           DateTime @default(now())
+  updatedAt           DateTime @updatedAt
+}