Stuck msgs if you bg the app before they send

### Steps to reproduce

1. Send some messages on slow connectivity
2. Bg the app rapidly
3. Reopen later
4. Discover that the messages are stuck at the bottom of the timeline, but are not shown as unsent
5. You have no idea if they actually sent or not (it did, though), so you end up resend it and end up with dups.

### Outcome

#### What did you expect?

Messages to not get stuck, even if the echo is delayed, or even if didn’t get a 200 from the send.

#### What happened instead?

Reliably flakey stuck messages since ~1.8.17

### Your phone model

12 pro max

### Operating system version

15.5

### Application version

1.8.18 but since 1.8.17 or so

### Homeserver

m.org

### Will you send logs?

No

Start by reproducing the issue with slow connectivity: send messages, background the app before the echo or 200 response, then reopen it. Trace the message-send and backgrounding behavior in the iOS app and verify whether the message is sent or unsent. Done means messages do not remain misleadingly stuck or cause duplicate resends.

Written by the indexing model from the issue text.