feat: resolve attachment user through cfw-auth binding

Co-Authored-By: Claude <noreply@anthropic.com>

src/problem.ts

@@ -0,0 +1,33 @@
+export class HttpProblem extends Error {
+  constructor(
+    readonly status: number,
+    readonly code: string,
+    message: string,
+    readonly detail: Record<string, unknown> = {},
+  ) {
+    super(message);
+  }
+}
+
+export function problemResponse(error: unknown): Response {
+  const problem =
+    error instanceof HttpProblem
+      ? error
+      : new HttpProblem(500, "internal-error", "Internal server error");
+
+  return Response.json(
+    {
+      type: `https://cfw-attachment.bowong.cc/problems/${problem.code}`,
+      title: problem.message,
+      status: problem.status,
+      detail: problem.detail,
+      code: problem.code,
+    },
+    {
+      status: problem.status,
+      headers: {
+        "content-type": "application/problem+json",
+      },
+    },
+  );
+}