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/Livewire/Project/Application/Rollback.php

@@ -23,10 +23,14 @@ class Rollback extends Component
     #[Validate(['integer', 'min:0', 'max:100'])]
     public int $dockerImagesToKeep = 2;
 
+    public bool $serverRetentionDisabled = false;
+
     public function mount()
     {
         $this->parameters = get_route_parameters();
         $this->dockerImagesToKeep = $this->application->settings->docker_images_to_keep ?? 2;
+        $server = $this->application->destination->server;
+        $this->serverRetentionDisabled = $server->settings->disable_application_image_retention ?? false;
     }
 
     public function saveSettings()