Enh: archiver does not specify id when adding entries to notifications

The archiver currently leaves it to ES to generate the id for notification entries.

This is presumably because there may be multiple references for a single message.
However the code ensures that there cannot be duplicate entries for the same cid from a single message.

So it looks like the code could use cid+mid for the id.

[This would allow re-import if necessary without creating additional entries, but that might only be useful for testing]

Locate the archiver code that adds notification entries and inspect how it indexes them in Elasticsearch. Confirm how cid and mid are available, then make the entry id use both values; done means repeated imports do not create duplicate entries for the same message reference.

Written by the indexing model from the issue text.