Web terminal: open a shell on a site from the dashboard

Hammerhead signs in to the site's Bamboo SSH server through the SSH port an edge
exposes, with a key of its own, and relays the shell to the browser over a
WebSocket (xterm.js, served by Hammerhead). The SSH dialog shows Hammerhead's
public key to add to authorized_keys. Host keys are saved on first use and a
changed one is refused; the terminal opens only from the dashboard's own
origin, is audited, and closes after 30 minutes idle. Errors reach the page as
messages over the socket. An end-to-end test runs a real shell through the
edge, tunnel and Bamboo.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

db.go

@@ -128,6 +128,7 @@ func (d *DB) sealed(v any) any {
 	case Settings:
 		x.SMTP.Pass = b.Seal(x.SMTP.Pass)
 		x.Sandbar.Token = b.Seal(x.Sandbar.Token)
+		x.Terminal.Key = b.Seal(x.Terminal.Key)
 		return x
 	}
 	return v
@@ -157,7 +158,7 @@ func (d *DB) unseal(v any) error {
 	case *AlertChannel:
 		return errors.Join(d.open1(&x.Target), d.open1(&x.Token))
 	case *Settings:
-		return errors.Join(d.open1(&x.SMTP.Pass), d.open1(&x.Sandbar.Token))
+		return errors.Join(d.open1(&x.SMTP.Pass), d.open1(&x.Sandbar.Token), d.open1(&x.Terminal.Key))
 	}
 	return nil
 }