Bulk, idempotent ingest of a device's blood pressure readings, by day

For Apple Health/Health Connect syncing. Each item replaces one source's readings for one local day, so sending a day again is a no-op and re-walking a history stores nothing twice: a reading is identified by its instant within its source. At most 500 days per request and 48 readings per day; a day whose readings can't all be stored is reported "rejected" on its own rather than failing the batch. Body Lab keeps every reading and says nothing about any of them.