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/users/[id]/route.ts

@@ -0,0 +1,117 @@
+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';
+
+type TunnelRow = {
+  user_id: string;
+  subdomain: string;
+  token: string;
+  is_active: boolean;
+  bytes_used: number;
+  quota_bytes: number;
+  last_seen_at: string | null;
+  created_at: string;
+};
+
+export async function GET(
+  _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 user id' }, { status: 400 });
+  }
+
+  const admin = getSupabaseAdmin();
+
+  const { data: userRes, error: userErr } =
+    await admin.auth.admin.getUserById(id);
+  if (userErr || !userRes.user) {
+    return NextResponse.json({ error: 'user not found' }, { status: 404 });
+  }
+  const u = userRes.user;
+
+  const { data: tunnel } = await admin
+    .from('tunnels')
+    .select(
+      'user_id, subdomain, token, is_active, bytes_used, quota_bytes, last_seen_at, created_at',
+    )
+    .eq('user_id', id)
+    .maybeSingle<TunnelRow>();
+
+  const { data: audit } = await admin
+    .from('admin_audit_log')
+    .select('id, actor_email, action, target_type, target_id, details, created_at')
+    .eq('target_id', id)
+    .order('created_at', { ascending: false })
+    .limit(25);
+
+  return NextResponse.json({
+    user: {
+      id: u.id,
+      email: u.email ?? null,
+      role: (u.app_metadata?.role as string | undefined) ?? 'user',
+      banned_until:
+        (u as unknown as { banned_until?: string | null }).banned_until ?? null,
+      email_confirmed_at: u.email_confirmed_at ?? null,
+      created_at: u.created_at,
+      last_sign_in_at: u.last_sign_in_at ?? null,
+    },
+    tunnel: tunnel
+      ? {
+          subdomain: tunnel.subdomain,
+          is_active: tunnel.is_active,
+          bytes_used: tunnel.bytes_used,
+          quota_bytes: tunnel.quota_bytes,
+          last_seen_at: tunnel.last_seen_at,
+          created_at: tunnel.created_at,
+        }
+      : null,
+    audit: audit ?? [],
+  });
+}
+
+export async function DELETE(
+  _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 user id' }, { status: 400 });
+  }
+  if (id === auth.user.id) {
+    return NextResponse.json(
+      { error: 'you cannot delete your own account' },
+      { status: 400 },
+    );
+  }
+
+  const admin = getSupabaseAdmin();
+
+  // Remove the tunnel row first (FK to auth.users).
+  await admin.from('tunnels').delete().eq('user_id', id);
+
+  const { error } = await admin.auth.admin.deleteUser(id);
+  if (error) {
+    return NextResponse.json({ error: error.message }, { status: 500 });
+  }
+
+  await logAdminAction(auth.user, {
+    action: 'user.delete',
+    target_type: 'user',
+    target_id: id,
+  });
+
+  return NextResponse.json({ ok: true });
+}