[Feature]: document change digest and follow notifications

[HMarzban]

Problem

There is no way to follow a document and receive a summary of what changed. None of the pieces exist: no change-digest module, and no content-change notification type.

What is missing before this can start

A user story. Specifically:

• Who follows a document, and how they opt in or out.
• What time window a digest covers.
• Whether the digest is per heading or per document.
• How a private document restricts the audience.

Acceptance

• A user story is written in this issue.
• Only then is implementation scope estimated.

Notes

Filed to record the gap. Do not start implementation from this text alone.