feat: add MariaDB support, admin API + admin UI persistence, image generation caching and missing menu images; move credentials to .env and add .env.example

.env.example

@@ -0,0 +1,12 @@
+# OpenAI API key
+OPENAI_KEY=your-openai-key-here
+
+# Database credentials
+DB_HOST=localhost
+DB_USER=your-db-user
+DB_PASSWORD=your-db-password
+DB_NAME=your-db-name
+
+# Admin credentials
+ADMIN_USER=your-admin-username
+ADMIN_PASSWORD=your-admin-password