App fails to recover rapidly from UISIs

### Steps to reproduce

1. A bad case of https://github.com/vector-im/element-web/issues/23113 made almost all my E2EE messages UISI.
2. To work around it, I opened up an old EW instance, which thankfully received all the megolm keys correctly and stored them to online backup.
3. However, looking at EW, it still showed all the messages as UISI, and we no longer provide a way to explicitly request decryption. Over the next hour or so, some of the resolved (showing grey shields as expected), but not all. I couldn't spot the pattern.

Apparently we only request keys from online backup (or keyshare?) when a message is first loaded. This makes the feature almost useless, given by definition you don't have the keys when the message is first loaded. I'm assuming other sessions are getting pulled in if I backapaginate?

### Outcome

#### What did you expect?

UISIs should try to resolve themselves from backup whenever they're shown to the user.

#### What happened instead?

UISIs only try to resolve when they are first received, making them very hard to flush out, especially now we don't have a manual "retry decryption" button any more, since https://github.com/matrix-org/matrix-react-sdk/pull/11027

### Operating system

_No response_

### Browser information

_No response_

### URL for webapp

_No response_

### Application version

develop

### Homeserver

_No response_

### Will you send logs?

No