Add per-application Docker image retention for rollback capability

Implement a per-application setting (`docker_images_to_keep`) in `application_settings` table to control how many Docker images are preserved during cleanup. The cleanup process now:

- Respects per-application retention settings (default: 2 images)
- Preserves the N most recent images per application for easy rollback
- Always deletes PR images and keeps the currently running image
- Dynamically excludes application images from general Docker image prune
- Cleans up non-Coolify unused images to prevent disk bloat

Fixes issues where cleanup would delete all images needed for rollback.

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

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

database/migrations/2025_12_05_000000_add_docker_images_to_keep_to_application_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('application_settings', function (Blueprint $table) {
+            $table->integer('docker_images_to_keep')->default(2);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('application_settings', function (Blueprint $table) {
+            $table->dropColumn('docker_images_to_keep');
+        });
+    }
+};