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/model/config.go

@@ -261,13 +261,17 @@ func Mask(p netip.Prefix) netip.Addr {
 // MAC address, or by its client identifier (option 61) when ClientID is
 // set. The subnet is the one whose prefix holds the address.
 type Reservation struct {
-	ID       int64      `json:"id"`
+	ID int64 `json:"id"`
+	// The client is known by its MAC address, its client identifier
+	// (option 61), or either when both are set.
 	MAC      string     `json:"mac,omitempty"`
 	ClientID string     `json:"client_id,omitempty"`
 	IP       netip.Addr `json:"ip"`
 	// Hostname is sent to the client (option 12).
 	Hostname string        `json:"hostname"`
 	Options  []OptionValue `json:"options"`
+	// LeaseTime overrides the subnet's lease time (0: inherit).
+	LeaseTime int `json:"lease_time,omitempty"`
 	// Disabled reservations are kept but ignored: the client is treated
 	// like any other, and an address inside a pool may go to others.
 	Disabled bool   `json:"disabled"`