【后端接口】邮件的定时投递：定时邮件的发送与撤回

---定时邮件：排定发送时刻
function TimedMail.schedule(actor, sendAt, title, body, reward)
    local mailId = 'TM_' .. os.time() .. '_' ..
        math.random(1000, 9999)
    TimedMail.queue[mailId] = {
        to = actor, title = title,
        body = body, reward = reward,
        sendAt = sendAt, status = 'pending',
    }
    sendmsg(actor, 1, "定时邮件已排定")
    return mailId
end

---撤回：未投递的邮件可撤回修改
function TimedMail.revoke(actor, mailId)
    local mail = TimedMail.queue[mailId]
    if not mail or mail.status ~= 'pending' then
        sendmsg(actor, 1, "邮件不在待发状态")
        return false
    end
    mail.status = 'revoked'
    sendmsg(actor, 1, "定时邮件已撤回")
    return true
end