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>

database/migrations/2025_12_05_100000_add_disable_application_image_retention_to_server_settings.php

@@ -0,0 +1,22 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('server_settings', function (Blueprint $table) {
+            $table->boolean('disable_application_image_retention')->default(false);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('server_settings', function (Blueprint $table) {
+            $table->dropColumn('disable_application_image_retention');
+        });
+    }
+};