Add server-level toggle to disable application image retention

Adds a new server-level setting that allows administrators to disable
per-application image retention globally for all applications on a server.
When enabled, Docker cleanup will only keep the currently running image
regardless of individual application retention settings.

Changes:
- Add migration for disable_application_image_retention boolean field
- Update ServerSetting model with cast
- Add checkbox in DockerCleanup page (Advanced section)
- Modify CleanupDocker action to check server-level setting
- Update Rollback page to show warning and disable inputs when server
  retention is disabled
- Add helper text noting server-level override capability

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/Models/ServerSetting.php

@@ -61,6 +61,7 @@ class ServerSetting extends Model
         'is_reachable' => 'boolean',
         'is_usable' => 'boolean',
         'is_terminal_enabled' => 'boolean',
+        'disable_application_image_retention' => 'boolean',
     ];
 
     protected static function booted()