Terminal: SSH to NAS host instead of container shell

- Rewrite terminal.py to use asyncssh instead of pty/fork
- Add SSH key pair for dashboard container auth
- Mount SSH key and add SSH config vars in docker-compose
- Install openssh-client in Dockerfile
- Add asyncssh to requirements.txt

dashboard/docker-compose.yml

@@ -30,8 +30,13 @@ services:
       - GITEA_URL=http://gitea:3000
       - GITEA_TOKEN=${GITEA_TOKEN}
       - VOLUME_ROOT=/volume1
+      - SSH_HOST=host.docker.internal
+      - SSH_USER=zjgump
     volumes:
       - /volume1:/volume1
+      - ./ssh/dashboard_terminal:/app/ssh/id_ed25519:ro
+    extra_hosts:
+      - "host.docker.internal:host-gateway"
     networks:
       - internal
       - gitea_gitea