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/Actions/Server/CleanupDocker.php

@@ -115,8 +115,10 @@ class CleanupDocker
             $applications = $server->applications();
         }
 
+        $disableRetention = $server->settings->disable_application_image_retention ?? false;
+
         foreach ($applications as $application) {
-            $imagesToKeep = $application->settings->docker_images_to_keep ?? 2;
+            $imagesToKeep = $disableRetention ? 0 : ($application->settings->docker_images_to_keep ?? 2);
             $imageRepository = $application->docker_registry_image_name ?? $application->uuid;
 
             // Get the currently running image tag