Feature: "Any" option for engagement trigger posts

[flintfromthebasement]

Summary

• Engagements can now fire for any course, membership, lesson, quiz, section, access plan, or course track — not just a specific one
• Addresses the feature request where N engagements were needed for N courses when the same email/certificate/achievement should fire for all of them
• Works for all trigger types: enrollment, purchase, completion, quiz pass/fail, etc.
• Covers courses and memberships (and all other post types) in one change

How It Works

Admin UI: The trigger post select fields (e.g., "Select a Course") can now be cleared. When cleared, the placeholder reads "Any Course" (or "Any Membership", etc.), and a help text says "Leave blank to apply to all."

Storage: An empty selection saves the literal string 'any' in _llms_engagement_trigger_post. This is an explicit opt-in — we intentionally avoided treating empty/zero values as wildcards so that existing engagements with accidentally blank trigger posts don't suddenly fire on everything.

Query: The engagement matching SQL adds OR relation_meta.meta_value = 'any' alongside the specific post ID match, so both targeted and wildcard engagements are returned.

List table: The engagements admin list shows "Any" in italics for wildcard engagements.

Design Decision: Explicit 'any' vs Empty-Means-Any

We considered treating empty or 0 values as "any" but rejected it because:

• Existing engagements with accidentally empty trigger posts would silently start firing on everything
• An explicit sentinel value ('any') makes the intent clear in the database
• It's safer for sites upgrading — no surprise behavior changes

Files Changed

File	Change
class.llms.engagements.php	SQL WHERE clause matches both specific post IDs and 'any'
class.llms.meta.box.engagement.php	Placeholder text, value loading, save logic (empty → 'any'), description text
class.llms.admin.post.table.engagements.php	Shows "Any" label for wildcard engagements

Duplicate/Spam Safety

• Achievements & certificates: The existing dupcheck() uses the actual related post ID (the course the student enrolled in), not the engagement's trigger post config. So an "any course" achievement is awarded once per course — not re-awarded for the same course.
• Emails: No dupcheck (by design) — they fire on every matching event, which is correct for welcome/notification emails.

Test Plan

• Create an engagement with trigger "Student enrolls in a course", leave course blank (shows "Any Course")
• Enroll a student in Course A → engagement fires
• Enroll same student in Course B → engagement fires again
• Create a second engagement for a specific Course A
• Enroll in Course A → both engagements fire
• Enroll in Course B → only the "any" engagement fires
• Verify "Any Membership" works the same way for membership triggers
• Verify the engagements list table shows "Any" for wildcard engagements
• Verify existing specific engagements are unaffected
• Verify achievements with "any" trigger only award once per course (dupcheck)