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/admin/validators.ts

@@ -0,0 +1,44 @@
+/**
+ * Small manual validators for admin API inputs (zod is intentionally not a
+ * dependency). Each returns a discriminated result.
+ */
+
+const UUID_RE =
+  /^[0-9a-f]{8}-[0-9a-f]{4}-[1-5][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i;
+
+export function isUuid(v: unknown): v is string {
+  return typeof v === 'string' && UUID_RE.test(v);
+}
+
+export function parseBoolean(v: unknown): boolean | null {
+  if (typeof v === 'boolean') return v;
+  return null;
+}
+
+export function parsePositiveInt(
+  v: unknown,
+  max: number,
+): { ok: true; value: number } | { ok: false; error: string } {
+  if (typeof v !== 'number' || !Number.isFinite(v) || !Number.isInteger(v)) {
+    return { ok: false, error: 'must be an integer' };
+  }
+  if (v <= 0) return { ok: false, error: 'must be positive' };
+  if (v > max) return { ok: false, error: `must be <= ${max}` };
+  return { ok: true, value: v };
+}
+
+export function parsePageParam(v: string | null, fallback = 1): number {
+  const n = v ? Number(v) : NaN;
+  if (!Number.isFinite(n) || n < 1) return fallback;
+  return Math.floor(n);
+}
+
+export function parsePerPageParam(
+  v: string | null,
+  fallback = 25,
+  max = 100,
+): number {
+  const n = v ? Number(v) : NaN;
+  if (!Number.isFinite(n) || n < 1) return fallback;
+  return Math.min(max, Math.floor(n));
+}