feat(notifications): send notifications to all post instructors

[faisalahammad]

Description

Notifications for courses and memberships currently target only the post author, ignoring secondary instructors and assistants stored in the _llms_instructors post meta. This adds an "All Instructors" recipient option on the 11 notifications that previously targeted the author or course author, and a per-course / per-membership override that lets instructors force the setting on or off for a single course or membership regardless of the global admin default.

For notifications triggered by a lesson, section, quiz, or order, the related course or membership is resolved by walking the related objects (Lesson::get_course, Quiz::get_course, Section::get_course, Transaction::get_order, Order::get_product) before reading the override meta and the instructor list.

Fixes #375

How has this been tested?

PHPUnit: vendor/bin/phpunit --filter 'Notification|Instructor' passes: 94 tests, 384 assertions, 0 failures.

PHPCS: composer run-script check-cs exits 0 with no errors on changed files.

Manual test plan:

1. Global "Send to all instructors"

   • Add a second user as an instructor to a test course.
   • Go to LifterLMS > Settings > Notifications > Enrollment.
   • Enable the new "All Instructors" checkbox under Email recipients.
   • Enrol a student in that course.
   • Result: both the author and the secondary instructor receive the enrolment email.

2. Per-course override (force off)

   • On the test course, set General > "Send Notifications to All Instructors" to "Do Not Send".
   • Enrol another student in the same course.
   • Result: neither instructor receives the enrolment email, even though the global setting is still on.

3. Per-course override (force on)

   • Turn the global notification setting off.
   • On a different course, set General > "Send Notifications to All Instructors" to "Send".
   • Enrol a student in that course.
   • Result: the course's instructors receive the enrolment email despite the global off.

4. Lesson triggered notification

   • Enable "All Instructors" globally for the Lesson Complete notification.
   • Add a secondary instructor to the lesson's parent course.
   • Mark the lesson complete for an enrolled student.
   • Result: both instructors receive the lesson-complete email.

5. Order triggered notification

   • Enable "All Instructors" globally for Purchase Receipt.
   • Add a secondary instructor to the product (course or membership).
   • Complete a purchase.
   • Result: both instructors receive the purchase receipt.

Environment: WordPress 6.x, PHP 7.4+, LifterLMS dev branch.

Screenshots

n/a (no visible UI changes for end users; the new control matches existing select styling on the Course Options and Membership Settings metaboxes).

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• This PR requires and contains at least one changelog file. (.changelogs/all-instructors-notifications.yml — added / minor)
• My code has been tested.
• My code passes all existing automated tests.
• My code follows the LifterLMS Coding & Documentation Standards.