Add backend environment example

backend/.env.example

@@ -0,0 +1,21 @@
+APP_ENV=development
+APP_HOST=127.0.0.1
+APP_PORT=3000
+PUBLIC_API_URL=https://api.example.ru
+PUBLIC_WS_URL=wss://api.example.ru/ws
+TRUST_PROXY=true
+
+DATABASE_PATH=./data/chat.sqlite
+
+ADMIN_ORIGIN=https://admin.example.ru
+WIDGET_ORIGIN=https://widget.example.ru
+
+TELEGRAM_BOT_TOKEN=
+TELEGRAM_PROXY_ENABLED=false
+TELEGRAM_PROXY_TYPE=socks5
+TELEGRAM_PROXY_HOST=127.0.0.1
+TELEGRAM_PROXY_PORT=9050
+TELEGRAM_PROXY_USERNAME=
+TELEGRAM_PROXY_PASSWORD=
+
+JWT_SECRET=change-me