fix: use api.upload() for artefact file attachments

api.post() was setting Content-Type to application/json and JSON.stringifying
the FormData, which destroyed the file data and caused "Either file upload or
drive_url is required" error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/src/pages/Artefacts.jsx

@@ -710,7 +710,7 @@ function ArtefactDetailPanel({ artefact, onClose, onUpdate, onDelete, projects =
     try {
       const formData = new FormData()
       formData.append('file', file)
-      await api.post(`/artefacts/${artefact.Id}/versions/${selectedVersion.Id}/attachments`, formData)
+      await api.upload(`/artefacts/${artefact.Id}/versions/${selectedVersion.Id}/attachments`, formData)
       toast.success('File uploaded')
       loadVersionData(selectedVersion.Id)
     } catch (err) {