User Avatar History

### Your use case

#### What would you like to do?

Be able to view previous user avatars easily.

#### Why would you like to do it?

Telegram has this feature, and it's useful when trying to recognise users with high-churn profile changes, such as avatars.

### Have you considered any alternatives?

_No response_

### Additional context

There is a high privacy hazard associated with this, even though the data is there (in DM rooms or public rooms) to support this, this feature would make it easier to access that data through one way or another, lowering the threshold.

Additionally, while this feature specifically focuses on Avatars, other details of a user are also encoded in the history of a room, and those details are less relevant for retrieval (display names, or in the future, bio). These elements have an even higher privacy risk.

Finally, for this feature, I think a user should be able to remove avatars from their "history". However, this is tricky, as room history is immutable, and the only way to hide data from state history is redaction, which is discouraged.

No implementation files, tests, or entry points are named. Start by clarifying the client scope, privacy constraints, and handling of avatar removal or redaction; the issue is ready for implementation only once those decisions produce concrete acceptance criteria.

Written by the indexing model from the issue text.