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/api/types.go

@@ -96,8 +96,10 @@ type ReservationInput struct {
 	IP       *string        `json:"ip,omitempty"`
 	Hostname *string        `json:"hostname,omitempty"`
 	Options  *[]OptionInput `json:"options,omitempty"`
-	Disabled *bool          `json:"disabled,omitempty"`
-	Comment  *string        `json:"comment,omitempty"`
+	// LeaseTime in seconds; 0 for the subnet's.
+	LeaseTime *int    `json:"lease_time,omitempty"`
+	Disabled  *bool   `json:"disabled,omitempty"`
+	Comment   *string `json:"comment,omitempty"`
 }
 
 // Subnet is a subnet with its usage.