Instant Slack alerts via Incoming Webhook with Block Kit rich formatting:
- New trainer registrations (name, role, org, business type, photo)
- Ticket purchases (purchaser, event, count, total, gateway)
- Events submitted by trainers via TEC Community Events form
- Events published by admins (draft/pending → publish)
Settings UI with webhook URL field, validation, and test button.
Non-blocking sends so Slack failures never affect user flows.
Atomic add_post_meta idempotency guards prevent duplicate sends.
Code reviewed by GPT-5 (Codex) — all 5 findings addressed.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
