GHSA-8rwr-f68v-cvw6

### Summary The upload-by-URL path did not enforce `NC_ATTACHMENT_FIELD_SIZE` against either the remote file's advertised `Content-Length` or the decoded length of a `data:` URI, allowing an authenticated user to bypass the configured per-file size limit.

### Details The attachments service now checks `NC_ATTACHMENT_FIELD_SIZE` against both the HEAD response's `content-length` and the decoded length of a `data:` URI body before fetching. The local storage plugin additionally sets `maxContentLength` on the axios download so a malicious server cannot stream past the limit.

### Impact Authenticated users with upload permission could attach files larger than the operator-configured limit, defeating storage and bandwidth caps.

### Credit This issue was reported by [@bugbunny-research](https://github.com/bugbunny-research).