Sort by article length (#8119)

* Sort by article length
fix https://github.com/FreshRSS/Extensions/issues/378
Very basic using simply SQL `LENGTH()` function.

<img width="492" height="217" alt="image" src="https://github.com/user-attachments/assets/7cf37303-76c8-4411-b8b1-075e81535b60" />

* Improve content length retrieval

app/Models/UserConfiguration.php

@@ -55,7 +55,7 @@ declare(strict_types=1);
  * @property bool $show_nav_buttons
  * @property 'big'|'small'|'none' $mark_read_button
  * @property 'ASC'|'DESC' $sort_order
- * @property 'id'|'c.name'|'date'|'f.name'|'link'|'title'|'rand' $sort
+ * @property 'id'|'c.name'|'date'|'f.name'|'link'|'title'|'rand'|'length' $sort
  * @property array<int,array<string,string>> $sharing
  * @property array<string,string> $shortcuts
  * @property bool $sides_close_article