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>

resources/views/livewire/server/docker-cleanup.blade.php

@@ -78,6 +78,10 @@
                                 <li>Networks not attached to running containers will be permanently deleted (networks used by stopped containers are affected).</li>
                                 <li>Containers may lose connectivity if required networks are removed.</li>
                             </ul>" />
+                        <x-forms.checkbox canGate="update" :canResource="$server" instantSave
+                            id="disableApplicationImageRetention"
+                            label="Disable Application Image Retention"
+                            helper="When enabled, Docker cleanup will delete all old application images regardless of per-application retention settings. Only the currently running image will be kept.<br><br><strong>Warning: This disables rollback capabilities for all applications on this server.</strong>" />
                     </div>
                 </div>
             </form>