fix(activity): use source document for comment mention notifications

[ericvicenti]

Summary

Fixes #693.

In ActivityFeed.ListEvents, comment mention events were incorrectly setting newMention.sourceDocument to the mention target. For account/profile mentions, that meant the source document became hm://<mentioned-account>/:profile, causing notification titles like mentioned you in :profile and causing web site-scoped inbox filtering to hide the notification.

This PR keeps newMention.target as the mentioned entity, but sets newMention.sourceDocument from the comment blob's resource IRI.

Changes

• Include the comment/source structural blob resource IRI in the mentions feed query.
• Use that source resource IRI for SourceDocument when the mention source blob is a Comment.
• Add a regression test covering a comment mention of hm://<account>/:profile inside hm://<author>/doc.

Validation

go test ./backend/api/activity/v1alpha