Show Tailscale device key expiry

Tailscale node keys expire (180 days by default unless disabled per
device) and an expired key drops the device off the tailnet until
re-authenticated -- worth surfacing before it happens.

adapter.listDevices() now reads `expires` and `keyExpiryDisabled` from
the device list API (`?fields=all`, already being fetched). Go's zero
time ("0001-01-01T00:00:00Z") is what Tailscale returns for "no real
expiry set" and is treated as null rather than shown as a bogus 1AD
date.

New "Key expiry" column on the Tailscale page: "Never" when disabled,
otherwise the date plus a badge (green/yellow/red matching the
Secrets module's ok/expiring/expired convention) using the same
30-day warning window as that module's default. The devices summary
gained `expiringSoon` (<=30 days left, including already-expired),
surfaced in the page header and as a new warning line on the
Dashboard's Tailscale widget, alongside the existing "awaiting
authorization" one.

Verified the parsing (real expiry, disabled/zero-time, already-expired,
far-future) against the compiled adapter with fetch calls to
api.tailscale.com redirected to a local mock server, since this
sandbox can't reach the user's real tailnet.

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

web/src/api/client.ts

@@ -310,11 +310,13 @@ export interface TailscaleDevice {
   isExitNode: boolean;
   authorized: boolean;
   online: boolean;
+  keyExpiry: string | null;
+  keyExpiryDisabled: boolean;
 }
 
 export interface TailscaleDevicesResponse {
   devices: TailscaleDevice[];
-  summary: { total: number; online: number; unauthorized: number };
+  summary: { total: number; online: number; unauthorized: number; expiringSoon: number };
 }
 
 export interface GiteaWorkflowRun {