feat: 授权延长——同事一键申请，管理员批准时决定时长；时长支持 30 分钟与自定义

同事端
- 用过邀请链接的设备再次打开链接时显示自己的授权状态：有效时可申请延长，
  访问结束（到期或通道关闭）后 2 小时内也可以；超过 2 小时或被撤销则提示重新获取邀请
- 「申请延长」只有一个按钮，提交后复用审批状态页，批准后显示新的到期时间
- 用过链接的设备不再自动重新放通，统一走申请延长

网关
- 授权结束后保留 2 小时的记录（通道关闭后仍保留，随状态文件持久化）；撤销的不能延长，
  删除通道 / 解散组时一并清除（unregister 带 deleted 标记）
- 新增 extend_lease：有效授权从原到期时间顺延，已结束的从现在起算，不受通道最长开启时长截断，
  并批准该设备的延长申请；指标里带上全部可延长设备与申请（包括已关闭的通道）
- 授权时长支持分钟数（"30m"、"90m"，5 分钟到 30 天）

Agent / App / 网页面板
- POST .../leases/extend：通道已关闭时先自动开启（组则全组开启），并让通道至少开到授权结束
- 授权管理新增「申请延长」「最近结束 · 2 小时内可延长」，已授权设备可直接「延长」；
  卡片、菜单栏和系统通知提示延长申请，菜单栏可直接选时长批准
- 邀请的可访问时长、通道最长开启时长增加 30 分钟与自定义；网页面板弹窗层级高于侧边抽屉
- 邀请分享文案提示「授权结束后 2 小时内可重新打开此链接申请延长」

README_EN.md

@@ -54,10 +54,11 @@ PortRelay turns that into three steps:
 **Access control**
 
 - Three access modes: **invite-only** (the default and safest), an IP / CIDR allowlist (which can be combined with invites), and open to all.
-- Each invite link has its own expiry, device limit, access duration once granted (1 h, 2 h, 8 h, until end of day, or until the tunnel closes) and an optional approval step.
+- Each invite link has its own expiry, device limit, access duration once granted (30 min, 1 h, 2 h, 8 h, until end of day, until the tunnel closes, or a custom length) and an optional approval step.
 - When approval is required, the app shows a system notification and you approve or deny with one click. The requester's page refreshes on its own.
 - **The access clock starts at the first connection**, so a teammate can accept an invite now and use it later.
 - When access expires or is revoked, **existing connections are dropped too**. Unauthorized connections are reset (TCP RST) and logged.
+- **Extensions.** Within two hours after access ends (even if the tunnel has been closed), a teammate reopens the original link and clicks 申请延长 (Request extension). You approve it by picking a duration in the app, the menu bar or the web panel, and a closed tunnel reopens on its own. You can also extend a running or just-ended grant yourself.
 - **Tunnel groups**: the add-tunnel form takes a list of targets, one host per row with comma-separated ports (a pasted list of IPs is split into rows), creates one tunnel per address on a contiguous block of gateway ports and puts them in one group. A group shares one invite link and one set of grants: a teammate is authorized once for the whole group, and one revoke disconnects them everywhere.
 
 **Tunnel lifecycle**