Use ephemeral SSH keys per rebuild instead of static config keys

Generate a fresh ed25519 key pair at rebuild time, inject the public key
into the Proxmox answer file, use the private key for cluster join over
SSH, then remove the key from both the remote host and the database.
This eliminates the need to manage static SSH keys in config/secrets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/store/hosts.go

@@ -109,6 +109,25 @@ func (s *Hosts) UpdateInfraID(ctx context.Context, id int64, infraHostID int64)
 	return err
 }
 
+func (s *Hosts) SetEphemeralKey(ctx context.Context, id int64, privateKey, publicKey string) error {
+	_, err := s.DB.ExecContext(ctx, `UPDATE hosts SET ssh_private_key = ?, ssh_public_key = ? WHERE id = ?`, privateKey, publicKey, id)
+	return err
+}
+
+func (s *Hosts) GetEphemeralKey(ctx context.Context, id int64) (privateKey, publicKey string, err error) {
+	var priv, pub sql.NullString
+	err = s.DB.QueryRowContext(ctx, `SELECT ssh_private_key, ssh_public_key FROM hosts WHERE id = ?`, id).Scan(&priv, &pub)
+	if err != nil {
+		return "", "", err
+	}
+	return priv.String, pub.String, nil
+}
+
+func (s *Hosts) ClearEphemeralKey(ctx context.Context, id int64) error {
+	_, err := s.DB.ExecContext(ctx, `UPDATE hosts SET ssh_private_key = NULL, ssh_public_key = NULL WHERE id = ?`, id)
+	return err
+}
+
 func (s *Hosts) Delete(ctx context.Context, id int64) error {
 	res, err := s.DB.ExecContext(ctx, `DELETE FROM hosts WHERE id = ?`, id)
 	if err != nil {