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

app/api/admin/audit/route.ts

@@ -0,0 +1,45 @@
+import { NextResponse, type NextRequest } from 'next/server';
+import { requireAdminApi } from '@/lib/auth/admin-guard';
+import { getSupabaseAdmin } from '@/lib/supabase/admin';
+import { parsePageParam, parsePerPageParam } from '@/lib/admin/validators';
+
+export const runtime = 'nodejs';
+export const dynamic = 'force-dynamic';
+
+export async function GET(req: NextRequest) {
+  const auth = await requireAdminApi();
+  if (!auth.ok) return auth.response;
+
+  const url = new URL(req.url);
+  const page = parsePageParam(url.searchParams.get('page'), 1);
+  const perPage = parsePerPageParam(url.searchParams.get('perPage'), 50, 100);
+  const action = (url.searchParams.get('action') ?? '').trim();
+  const targetType = (url.searchParams.get('target_type') ?? '').trim();
+
+  const admin = getSupabaseAdmin();
+
+  let query = admin
+    .from('admin_audit_log')
+    .select(
+      'id, actor_id, actor_email, action, target_type, target_id, details, created_at',
+      { count: 'exact' },
+    );
+  if (action) query = query.eq('action', action);
+  if (targetType) query = query.eq('target_type', targetType);
+
+  const from = (page - 1) * perPage;
+  const to = from + perPage - 1;
+  query = query.order('created_at', { ascending: false }).range(from, to);
+
+  const { data, error, count } = await query;
+  if (error) {
+    return NextResponse.json({ error: error.message }, { status: 500 });
+  }
+
+  return NextResponse.json({
+    entries: data ?? [],
+    total: count ?? (data?.length ?? 0),
+    page,
+    perPage,
+  });
+}