Add default orphan cleanup configuration

Problem/Motivation

The module currently does not provide default configuration values for orphan cleanup settings during installation.

Without default configuration, cleanup-related settings may be missing until manually configured, leading to inconsistent behavior and incomplete configuration exports.

Steps to reproduce

1. Install the Block Content Explorer module on a fresh Drupal site
2. Inspect the block_content_explorer.settings configuration object
3. Observe that orphan cleanup configuration values are not initialized by default

Proposed resolution

• Create config/install/block_content_explorer.settings.yml
• Add default values for orphan cleanup configuration settings
• Set:
  • auto_cleanup_enabled: true
  • cleanup_interval_days: 15
  • max_blocks_per_cron: 50
• Ensure configuration is automatically imported during module installation

Remaining tasks

• Create default configuration file
• Add orphan cleanup default values
• Test fresh module installation
• Verify configuration values using drush cget

User interface changes

None.

API changes

Adds default configuration for:

block_content_explorer.settings

Data model changes

None.