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/dhcpd/config.go

@@ -125,13 +125,11 @@ type client struct {
 	id  string // client identifier (option 61) in hex, or ""
 }
 
-// matches reports whether a reservation is for the client: by client
-// identifier when the reservation has one, else by MAC address.
+// matches reports whether the reservation is the client's: by its client
+// identifier or its MAC address, either one when the reservation has both
+// (a dual-boot machine sends different client IDs from one MAC address).
 func (cl client) matches(r *model.Reservation) bool {
-	if r.ClientID != "" {
-		return r.ClientID == cl.id
-	}
-	return r.MAC == cl.mac
+	return r.ClientID != "" && r.ClientID == cl.id || r.MAC != "" && r.MAC == cl.mac
 }
 
 func (cl client) String() string {