fix: reject failed attachment downloads

[vycdev2]

Summary

• reject non-successful HTTP responses before saving Discord attachment bodies
• surface the HTTP status through existing caller error handling instead of returning a path to an invalid image

Verification

• npm run build — passed (tsc)
• local HTTP behavior check — passed: HTTP 404 rejected without writing a file; HTTP 200 still saved the response body
• git diff --check — passed

Risk

• Low: the change only converts non-2xx attachment responses from false successes into errors already handled by callers.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.