feat(admin): renew a Free instance from the portal

Available from seven days before expiry and, deliberately, at any point
after it up to deletion, so the same button rescues a lapsed instance.

Renewal is manual because it is the entire reclaim signal: an instance
nobody renews is one nobody is using, which is what makes reaping safe.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

admin/internal/models/models.go

@@ -53,6 +53,13 @@ const MaxRelinksPerTerm = 3
 // paying customer's instance goes read-only.
 const GracePeriod = 3 * 24 * time.Hour
 
+// RenewWindow is how long before expiry a Free instance may be renewed.
+//
+// Renewal stays available after expiry too, right up until the reaper takes the
+// instance, so the same button rescues a lapsed instance instead of needing a
+// second mechanism.
+const RenewWindow = 7 * 24 * time.Hour
+
 type Account struct {
 	ID               bson.ObjectID `bson:"_id,omitempty" json:"-"`
 	AccountID        string        `bson:"account_id" json:"account_id"`