Photo attachment filename expression is evaluated at a different time than field expressions, causing mismatched names

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### Steps to reproduce

1. In QGIS Project Properties
Configure Photo attachments → File name expression to:
```
replace(@layer_name,' ','') || '-' || @mergin_username || '-' ||
format_date(now(),'yyyyMMdd_HHmmss')
```

2. Add a text field photo_url whose default/value expression is:
```
'/CORPORATE-DRIVE/' ||
replace(@layer_name,' ','') || '-' || @mergin_username || '-' ||
format_date(now(),'yyyyMMdd_HHmmss')
```

3. Create a new feature and take a photo.

### Actual result

The saved photo filename and photo_url differ by a few seconds, indicating the two expressions are evaluated at different times.

### Expected result

Both should match when they are logically based on the same expression.

### Why it matters

Downstream fields that depend on the photo name (e.g., paths/URLs) become inconsistent.

Start by reproducing the mismatch using the Photo attachments file name expression and the photo_url field's default/value expression in QGIS Project Properties. Trace where each expression is evaluated during feature creation and photo capture, then verify that both filenames use the same timestamp when the reproduction steps are repeated.

Written by the indexing model from the issue text.