Reservation lease time; reservations by MAC address or client ID

- A reservation can have its own lease time (--lease-time, the
  reservation dialog), over the subnet's. Schema migration 6.
- A reservation with a MAC address and a client ID matches either one
  (a dual-boot machine sends two client IDs from one MAC); before, it
  was found by MAC and then refused. A lease of the reserved address by
  the client's other identity counts as its own.
- The Leases page shows a client's client ID; reserving a lease uses
  both, or one with --by / the dialog's choice.

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

internal/server/h_leases.go

@@ -122,14 +122,38 @@ func (s *Server) reserveLease(w http.ResponseWriter, r *http.Request, p *auth.Pr
 	var in struct {
 		Hostname *string `json:"hostname,omitempty"`
 		Comment  string  `json:"comment,omitempty"`
+		// By is how the reservation knows the client: mac, client_id or
+		// both (the default when the client sent a client ID).
+		By string `json:"by,omitempty"`
 	}
 	if err := decodeOptional(r, &in); err != nil {
 		return err
 	}
 	l := s.dhcp.Lease(ip)
-	if l == nil || l.MAC == "" {
+	if l == nil || l.MAC == "" && l.ClientID == "" {
 		return notFound("no lease for %s", ip)
 	}
+	mac, cid := l.MAC, l.ClientID
+	switch strings.ReplaceAll(in.By, "-", "_") {
+	case "", "both":
+	case "mac":
+		cid = ""
+	case "client_id":
+		mac = ""
+	default:
+		return badRequest("by %q: use mac, client_id or both", in.By)
+	}
+	if mac == "" && cid == "" {
+		what := "client ID (option 61); reserve it by mac"
+		if in.By == "mac" {
+			what = "MAC address; reserve it by client_id"
+		}
+		return badRequest("the client of %s sent no %s", ip, what)
+	}
+	who := mac
+	if cid != "" {
+		who = strings.TrimSpace(mac + " client id " + cid)
+	}
 	host := l.Hostname
 	if in.Hostname != nil {
 		host = *in.Hostname
@@ -137,9 +161,9 @@ func (s *Server) reserveLease(w http.ResponseWriter, r *http.Request, p *auth.Pr
 		host = ""
 	}
 	var id int64
-	return s.change(w, r, p, "reserve "+ip.String()+" for "+l.MAC, func(st *model.State, newID func() int64) error {
+	return s.change(w, r, p, "reserve "+ip.String()+" for "+who, func(st *model.State, newID func() int64) error {
 		id = newID()
-		st.Reservations = append(st.Reservations, &model.Reservation{ID: id, MAC: l.MAC, IP: ip, Hostname: host, Options: []model.OptionValue{}, Comment: in.Comment})
+		st.Reservations = append(st.Reservations, &model.Reservation{ID: id, MAC: mac, ClientID: cid, IP: ip, Hostname: host, Options: []model.OptionValue{}, Comment: in.Comment})
 		return nil
 	}, func(st *model.State) any {
 		if res := st.ReservationByID(id); res != nil {