Statistics: Sum of new during period is 0 for custom date range

### Current Behavior

When selecting a custom date range for the Statistics graph the "New during range" cells always show 0 :(

### Steps to Reproduce

1. Open "Statistics" in admin
2. Select a custom date range
3. Observe the value fields in the mini stats table on top left

### Expected Behavior

Selecting a custom date range should calculate the sum of new items per each data type (user, discussions, posts) that occured during that custom range. This would be consistent with how this Statistics works with the pre-defined date ranges.

### Screenshots

### Environment

- Flarum version: 1.8.19 (I don't have a 2.0.0-rc.7 community with sufficient historic data. Worth checking on Discuss' Statistics

### Output of `php flarum info`

_No response_

### Possible Solution

_No response_

### Additional Context

_No response_

Start from the Statistics area in the admin interface and trace how the custom date range feeds the mini stats table's "New during range" values. Reproduce the issue with a custom range containing known users, discussions, and posts, then verify each data type reports the items created within that range rather than zero.

Written by the indexing model from the issue text.