Generate presigned upload URL for vault file

curl --request POST \
  --url https://api.case.dev/vault/{id}/upload \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "filename": "<string>",
  "contentType": "<string>",
  "metadata": {},
  "auto_index": true,
  "sizeBytes": 123
}
'

{
  "objectId": "cm123abc456",
  "uploadUrl": "https://vault-bucket.s3.amazonaws.com/objects/cm123abc456/contract.pdf?X-Amz-Algorithm=...",
  "expiresIn": 3600,
  "s3Key": "objects/cm123abc456/contract.pdf",
  "auto_index": true,
  "instructions": {
    "method": "PUT",
    "headers": {
      "Content-Type": "application/pdf"
    },
    "note": "Upload your file to the uploadUrl using PUT request. After upload completes, call POST /vault/:vaultId/ingest/:objectId to trigger processing."
  },
  "next_step": "POST /vault/vault_789/ingest/cm123abc456"
}