【数据存储】排行榜快照的历史回放：按周归档查询

---每周一零点归档上周榜单
function boardArchiveWeekly()
    local week = os.date("%Y_W%W", os.time() - 7 * 86400)
    for name, board in pairs(BOARDS) do
        local snap = {
            week = week, name = name,
            top50 = { table.unpack(board.list, 1, 50) },
            at = os.time(),
        }
        QF_PersistJson("board_archive_" .. name .. "_" .. week, snap)
    end
end

---查询某周某榜单的前十
function boardHistory(name, week)
    local snap = QF_LoadJson("board_archive_" .. name .. "_" .. week)
    if not snap then return {} end
    local out = {}
    for i = 1, math.min(10, #snap.top50) do
        out[i] = { rank = i, id = snap.top50[i].id, score = snap.top50[i].score }
    end
    return out
end