Added automatic detection to alert users when they're using the old
SSH-in-container method for temperature monitoring so they can upgrade
to the secure proxy architecture.
**Detection Logic:**
- Checks if Pulse is running in a container (Docker or LXC)
- Checks if SSH keys exist in data directory (/etc/pulse/.ssh)
- Checks if pulse-sensor-proxy socket is NOT available
- Sets legacySSHDetected and recommendProxyUpgrade flags in health endpoint
**API Changes:**
- Added fields to HealthResponse:
- legacySSHDetected: true when old method detected
- recommendProxyUpgrade: true when upgrade is recommended
- proxyInstallScriptAvailable: always true
**Use Case:**
Users who set up temperature monitoring before the proxy feature
won't know they should upgrade. This detection allows the frontend
to show a banner prompting them to re-run the setup script to
migrate to the secure proxy architecture.
**Frontend Integration (to be added):**
Frontend can poll /api/health and show a dismissible banner similar
to UpdateBanner when legacySSHDetected is true, with a button to
view the setup script.
Addresses #123
