fix(relationships): clean up post dependencies on permanent delete

[faisalahammad]

Description

Cleans up stale cross-post references when LifterLMS posts are permanently deleted. Previously, deleting a course, section, lesson, or membership left behind prerequisite links, membership auto-enroll entries, restriction arrays, parent meta, and the sitewide membership option pointing at the deleted post.

Hooks into the existing delete_post flow in LLMS_Post_Relationships (same place access plans and quiz/question cleanup already run). Adds a remove_from_meta action for serialized array meta and a get_posts_with_meta_containing() helper with a LIKE pre-filter.

Cleanup paths added

Course deleted

• Unset prerequisites on other courses that required it
• Remove course ID from membership auto-enroll arrays
• Force-delete child sections
• Unset parent-course meta on lessons

Section deleted

• Unset parent-section meta on lessons

Membership deleted

• Remove membership ID from post restriction arrays (_llms_restricted_levels)
• Remove membership ID from access plan availability restrictions (_llms_availability_restrictions)
• Clear sitewide lifterlms_membership_required option when it matches

Out of scope (this PR)

• Trash/untrash (wp_trash_post) — cleanup runs on permanent delete only, matching existing relationship handlers
• Cancel enrollments
• Track prerequisite cleanup
• Builder empty-node warnings

How has this been tested?

• New PHPUnit coverage in tests/phpunit/unit-tests/class-llms-test-post-relationships.php for each cleanup path
• Manual verification checklist covers course/section/membership delete cases

Screenshots / Screencasts

N/A — backend relationship cleanup only.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code has been tested.
• My code follows the WordPress/LifterLMS Coding Standards.
• My code follows the above Contributor Guide.
• My code includes any required tests that prove my fix is effective or that my feature works.
• My changelog has been updated to reflect the nature of my contribution.
• New files have the correct header and license information.
• I have read the CONTRIBUTING document.
• My code follows the WordPress Coding Standards.
• My code is ready to be merged.

Fixes #263