Missing migration for CoderedPage.index_order_by default/choices

## Summary
`coderedcms.models.page_models.CoderedPage.index_order_by` was updated (default/choices) in 6.0.0, but there is no migration for the change. Django reports a missing migration and `makemigrations --check --dry-run` generates one.

## Environment
- coderedcms 6.0.0
- Django 5.2.9
- Python 3.11.4

## Repro
```
pipenv run python manage.py makemigrations --check --dry-run
```

## Output
```
Migrations for 'coderedcms':
.venv/lib/python3.11/site-packages/coderedcms/migrations/0045_alter_coderedpage_index_order_by.py
~ Alter field index_order_by on coderedpage
```

## Notes
In `coderedcms/models/page_models.py`, `index_order_by` now includes an empty choice and uses `default = ""` with `blank=True`. The latest migration (0044) still reflects the old default and choices, so Django sees a model/migration mismatch.

Is it possible to include the missing `AlterField` migration in the next release?

coderedcms/models/page_models.py と migration 0044 から始め、次に `pipenv run python manage.py makemigrations --check --dry-run` を実行して、報告されたモデルと migration の不一致を調査します。`CoderedPage.index_order_by` のデフォルト値と choices が migration に反映され、チェックで migration が生成されなくなれば完了です。

索引モデルが issue の本文から書いたものです。