feat: configurable stop grace period for applications

Adds stop_grace_period to application settings (seconds, 1-3600, default 30).
Used in place of the hardcoded docker stop -t 30 in the four places that
stop application containers: rolling update shutdown, manual stop, stop on
another server, and preview deployment stop.

Non-positive values fall back to the default via ($val > 0) ? $val : default,
with tests covering 0 and -10 so the cast does not blow up if a bad value
ever lands in the db.

Picks up Jack Coy's work from #7125 which went dormant. His commits are
squashed here with credit below.

Co-authored-by: Jack Coy <jackman3000@gmail.com>

app/Models/ApplicationSetting.php

@@ -26,6 +26,7 @@ class ApplicationSetting extends Model
         'is_git_lfs_enabled' => 'boolean',
         'is_git_shallow_clone_enabled' => 'boolean',
         'docker_images_to_keep' => 'integer',
+        'stop_grace_period' => 'integer',
     ];
 
     protected $fillable = [