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/Dockerfile

@@ -8,6 +8,7 @@ RUN npm run build
 
 # Stage 2: Python runtime
 FROM python:3.12-slim
+RUN apt-get update && apt-get install -y --no-install-recommends openssh-client && rm -rf /var/lib/apt/lists/*
 WORKDIR /app
 COPY backend/requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt