refactor: normalize admin token handling

lib/admin-auth.ts

@@ -0,0 +1,37 @@
+import { timingSafeEqual } from "crypto";
+
+const ADMIN_TOKEN = process.env.SHAPE_ADMIN_TOKEN ?? "";
+
+type Headers = Record<string, string | string[] | undefined>;
+
+function getHeader(headers: Headers, name: string): string | undefined {
+  const raw = headers[name.toLowerCase()];
+  if (Array.isArray(raw)) return raw[0];
+  return raw;
+}
+
+/**
+ * True when the request is an authenticated admin call.
+ *
+ * Accepts either `Authorization: Bearer <token>` or `X-Admin-Token: <token>`.
+ * In development we allow the call when no token is configured so the
+ * local demo path keeps working.
+ */
+export function isAdmin(headers: Headers): boolean {
+  const bearer = getHeader(headers, "authorization")?.replace(/^Bearer\s+/i, "");
+  const xAdmin = getHeader(headers, "x-admin-token");
+
+  if (bearer) {
+    const a = Buffer.from(bearer);
+    const b = Buffer.from(ADMIN_TOKEN);
+    if (a.length !== b.length) return false;
+    return timingSafeEqual(a, b);
+  }
+
+  if (xAdmin) {
+    return xAdmin === ADMIN_TOKEN;
+  }
+
+  // Local-dev fallback: no token configured means demo mode.
+  return process.env.NODE_ENV !== "production";
+}