Bulk file uploads on POST /alert and PUT /alert

Both POST /alert and PUT /alert now accept a files array of base64-encoded attachments (and a file_action field on update). This brings the API to parity with the UI's bulk attachment flow.

curl -X PUT 'https://api.doppel.com/v1/alert?id=ORG-123' \
  -H 'Content-Type: application/json' \
  -H 'x-api-key: <your-api-key>' \
  -d '{
    "file_action": "upload",
    "files": [
      { "file_name": "evidence.png", "file_to_upload": "<base64>" }
    ]
  }'