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/tunnels/[id]/reset-usage/route.ts

@@ -0,0 +1,44 @@
+import { NextResponse, type NextRequest } from 'next/server';
+import { requireAdminApi } from '@/lib/auth/admin-guard';
+import { getSupabaseAdmin } from '@/lib/supabase/admin';
+import { logAdminAction } from '@/lib/auth/audit';
+import { isUuid } from '@/lib/admin/validators';
+
+export const runtime = 'nodejs';
+export const dynamic = 'force-dynamic';
+
+export async function POST(
+  _req: NextRequest,
+  { params }: { params: { id: string } },
+) {
+  const auth = await requireAdminApi();
+  if (!auth.ok) return auth.response;
+
+  const { id } = params;
+  if (!isUuid(id)) {
+    return NextResponse.json({ error: 'invalid tunnel id' }, { status: 400 });
+  }
+
+  const admin = getSupabaseAdmin();
+  const { data, error } = await admin
+    .from('tunnels')
+    .update({ bytes_used: 0 })
+    .eq('id', id)
+    .select('subdomain')
+    .maybeSingle<{ subdomain: string }>();
+  if (error) {
+    return NextResponse.json({ error: error.message }, { status: 500 });
+  }
+  if (!data) {
+    return NextResponse.json({ error: 'tunnel not found' }, { status: 404 });
+  }
+
+  await logAdminAction(auth.user, {
+    action: 'tunnel.reset_usage',
+    target_type: 'tunnel',
+    target_id: id,
+    details: { subdomain: data.subdomain },
+  });
+
+  return NextResponse.json({ ok: true });
+}