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/backend/requirements.txt

@@ -7,3 +7,4 @@ psutil==6.1.0
 pyjwt==2.8.0
 passlib==1.7.4
 pyotp==2.9.0
+asyncssh==2.17.0