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>

app/Models/ApplicationSetting.php

@@ -25,6 +25,7 @@ class ApplicationSetting extends Model
         'is_git_submodules_enabled' => 'boolean',
         'is_git_lfs_enabled' => 'boolean',
         'is_git_shallow_clone_enabled' => 'boolean',
+        'docker_images_to_keep' => 'integer',
     ];
 
     protected $guarded = [];