Cancel batch documents

Body for POST /api/v2/batches/{batch_id}/cancel/ (AD-178).

Cancels one, several, or all PENDING / PROCESSING BatchDocument rows in a batch. Already-COMPLETED docs are NEVER touched (work and credits already spent). The cancel endpoint is intentionally idempotent: re-posting when nothing is cancellable returns 200 with cancelled_doc_count == 0.

Cancel semantics:

• Per-row cancel (document_ids provided): only the listed docs are flipped to CANCELLED. The parent Batch.status is left untouched — the batch can still complete on its remaining docs.
• Full-batch cancel (document_ids omitted or empty): every non-terminal doc is flipped to CANCELLED AND Batch.status is set to CANCELLED (terminal). Subsequent reprocess + re-add are blocked by the terminal-batch guard in those endpoints.

For docs already in PROCESSING at cancel time, the worker's in-flight Lynx gRPC call is NOT aborted mid-stream — the call finishes and the extraction result is saved on the attachment. But the worker's terminal status update uses filter(status=PROCESSING) so a doc that flipped to CANCELLED in the meantime will silently skip the COMPLETED transition and stay CANCELLED. Credits already deducted are NOT refunded.

Fields: document_ids: Optional list of BatchDocument PKs to cancel. When omitted (or empty), every non-terminal doc in the batch is cancelled and the batch itself transitions to CANCELLED.

Response shape for POST /api/v2/batches/{batch_id}/cancel/.

Fields: batch: Updated parent batch (status flipped to CANCELLED on full-batch cancel; unchanged on per-row cancel). cancelled_doc_count: Number of BatchDocument rows whose status actually flipped to CANCELLED on this call. Zero when the request matched no cancellable docs (still 200 — idempotent). full_batch: True when this call cancelled the whole batch (document_ids was omitted/empty), False for per-row cancel.