Synchronously appends one or more JSONL records to an existing V4 dataset. Unlike the file-upload flow, this endpoint processes the whole payload within the request and returns the accepted/rejected counts directly — there is no async import job to poll.

Request body: JSONL — one JSON object per line — sent with a JSONL content type (e.g. application/x-ndjson). Blank lines are skipped. CSV is not supported on this endpoint.

Each record is validated against the dataset's current schema. Valid records are persisted as datapoints and sort after every existing datapoint. Records that are malformed or violate the schema are rejected individually and reported in errors — they do not fail the whole request. Re-appending an identical record is idempotent (counted as accepted, not written twice).

Limits and preconditions:

• At most 1000 records per request. Larger imports should use the file-upload flow.
• The dataset must be V4 and have a schema defined.
• Rejected with 409 Conflict while a dataset import or a schema migration is in progress.

If the dataset is attached to any batches with auto_sync_enabled, a sync is triggered automatically for the newly written datapoints.