'Unused screenshot' alert should wait a bit

### Describe the issue

I appreciate the messages about Unused screenshots. They are useful when a string is updated and the screenshot is no longer needed, and also when I get new screenshots with merge requests.

BUT for the love of the goddess, when I am uploading a screenshot, please give me a couple of minutes, at least, to find the strings before alerting me of an unused screenshot!

Each time I upload screenshots I get my mailbox bombarded with 'Unused screenshots' alerts. Please trigger the alert when an unused screenshot is older than 2 minutes.

Thanks!

### I already tried

- [x] I've read and searched [the documentation](https://docs.weblate.org/).
- [x] I've searched for similar filed issues in this repository.

### Steps to reproduce the behavior

1. On a component you manage, add a screenshot
2. Immediately after you add it, you receive an email of Unused Screenshot

### Expected behavior

You should be able to link the strings without any emails or alerts. And if you foget the screenshot, 5 minutes later you receive an alert.

### Screenshots

_No response_

### Exception traceback

```pytb

```

### How do you run Weblate?

weblate.org service

### Weblate version

weblate-2026.8.1-302-gad9f48f730

### Weblate deploy checks

```shell

```

### Additional context

_No response_

No files, tests, or entry points are named. Start by locating the code that generates the unused-screenshot email or alert and checking where screenshot age is evaluated. Done means newly uploaded screenshots are not alerted immediately, while screenshots still unused after the agreed delay produce an alert.

Written by the indexing model from the issue text.