Bulk, idempotent ingest of a device's daily readings

For Apple Health/Health Connect syncing: each item upserts by (source, measurement, date), so a retried or re-synced request is safe. At most 500 items; an unsupported measurement or an implausible value is reported "rejected" per item rather than failing the whole batch. Writes the way a connected sync does, then re-resolves each day it touched once.