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/migrations/20260117000100_add_users_and_system_config/migration.sql

@@ -0,0 +1,24 @@
+-- CreateTable
+CREATE TABLE "User" (
+    "id" TEXT NOT NULL PRIMARY KEY,
+    "username" TEXT,
+    "email" TEXT,
+    "passwordHash" TEXT NOT NULL,
+    "role" TEXT NOT NULL DEFAULT 'USER',
+    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" DATETIME NOT NULL
+);
+
+-- CreateTable
+CREATE TABLE "SystemConfig" (
+    "id" TEXT NOT NULL PRIMARY KEY DEFAULT 'default',
+    "registrationEnabled" BOOLEAN NOT NULL DEFAULT false,
+    "createdAt" DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" DATETIME NOT NULL
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "User_username_key" ON "User"("username");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "User_email_key" ON "User"("email");