Apply an upload to a dataset

Step two of the upload flow. Reads the file you uploaded and folds it into the dataset's current parquet.

• append — concatenate the new rows onto the existing data. Columns are unioned, so a schema mismatch produces nulls rather than an error.
• replace — discard the existing data and use the upload as the new dataset.
• delete — deletes the entire dataset: every S3 object under its prefix, the DynamoDB record and the team link. mutation_data_url is ignored. DELETE /dataset/{dataset_id} does exactly the same thing and says so in its name; prefer it.

mutation_data_url is the presigned URL with its query string removed (or any s3:// or https://<bucket>.s3.amazonaws.com/<key> URL). .csv, .tsv and .parquet are recognised by extension.

On success a dataset_update trigger is posted so workflows consuming this dataset can react.