Bulk Operations / Syncing

With the addition of storing per-item state, readers will likely want a way to mark large numbers of items as read in a single request. Both for offline reading, as well as just for efficiency's sake while online.

[Feedly's Markers API](https://developer.feedly.com/v3/markers/#mark-one-or-multiple-articles-as-read) allows operations to be made on one or more items at a time. We can likely do something similar, possibly by just including multiple `url[]` parameters in the request.

Start with the issue text and the linked Feedly Markers API to determine the intended bulk-marking request shape. Define the behavior for marking one or multiple items, including offline and online use cases; the work is done when the Microsub specification documents the agreed operation and parameters.

Written by the indexing model from the issue text.