Clean up deleted items from the database

We need to hold on to deleted items for a while in order to give all devices a chance to sync the deletions and remove the local copies of these items. After a while however, we could clean these up and remove them from the server entirely.

At the moment, we don't do this and so deleted items persist (E2E encrypted) on the server until the Sync account itself is deleted.

Start by tracing how the Go sync server records and serves deleted items, then determine an appropriate retention period that allows devices to synchronize deletions. Done means deleted items remain available during that period and are removed from the server afterward, without removing newer deletions prematurely.

Written by the indexing model from the issue text.