添加 Dex 认证配置和初始化脚本，创建 .env 示例文件，更新 docker-compose 配置以支持 OIDC 认证

.env.example.dex

@@ -0,0 +1,17 @@
+OIDC_ISSUER_URL=http://localhost:5556/.well-known/openid-configuration
+OIDC_CLIENT_ID=excalidraw
+OIDC_CLIENT_SECRET=excalidraw-secret
+OIDC_REDIRECT_URL=http://localhost:3002/auth/oidc/callback
+
+ADMIN_USERNAME=admin
+ADMIN_PASSWORD=your_secure_password
+ADMIN_EMAIL=[email protected]
+
+JWT_SECRET=your_super_secret_jwt_string
+
+STORAGE_TYPE=sqlite
+DATA_SOURCE_NAME=excalidraw.db
+LOCAL_STORAGE_PATH=./data
+
+OPENAI_API_KEY=sk-your_openai_api_key
+OPENAI_BASE_URL=https://api.openai.com