Request body limit

JSON request bodies are capped at 64 KiB. A larger body is rejected with 400 BAD_REQUEST before any JSON parsing happens. POST /v1/batches is the exception, in both its forms: its JSON body is capped at 512 KiB so that 5 000 numbers always fit, and its multipart/form-data form accepts a 2 MiB file and answers 413 FILE_TOO_LARGE beyond it.