feat(admin): comprehensive admin interface (users, tunnels, metrics, audit, reserved subdomains)

Adds an authenticated admin surface gated by auth.users.app_metadata.role==='admin'.

- lib/auth/admin-guard.ts: requireAdmin() (pages) + requireAdminApi() (routes)
- middleware.ts: defense-in-depth /admin and /api/admin guarding
- API: users (list/detail/role/ban/delete), tunnels (list + active/quota/reset/reassign/regenerate-token/delete), metrics, audit log, reserved subdomains
- Self-lockout prevention (no self demote/ban/delete)
- Best-effort Redis kill-switch via dependency-free net-socket client (REDIS_URL)
- admin_audit_log + reserved_subdomains migration (RLS on, service-role only)
- Admin UI (overview, users, tunnels, reserved, audit) + conditional nav link

lib/format.ts

@@ -0,0 +1,19 @@
+export function formatBytes(n: number): string {
+  const num = Number(n) || 0;
+  if (num < 1024) return `${num} B`;
+  const units = ['KiB', 'MiB', 'GiB', 'TiB', 'PiB'];
+  let v = num / 1024;
+  let i = 0;
+  while (v >= 1024 && i < units.length - 1) {
+    v /= 1024;
+    i++;
+  }
+  return `${v.toFixed(2)} ${units[i]}`;
+}
+
+export function formatDate(s: string | null | undefined): string {
+  if (!s) return '—';
+  const d = new Date(s);
+  if (Number.isNaN(d.getTime())) return '—';
+  return d.toLocaleString();
+}