docs(phase5b): self-paced enrollment documentation + website integration contract

- PHASE5B_SELF_PACED_ENROLLMENT.md: the full design doc.
- RUNBOOK_SELF_PACED_ENROLLMENT.md: operational runbook for /admin -> Programs.
- WEBSITE_ENROLLMENT_INTEGRATION.md: the complete contract for the separate website team —
  endpoint, auth, request/response schema, idempotency/retry rules, student matching, minor/
  no-email behavior, error responses, what not to send, local testing, production-secret
  handling. Written to be usable without reading the Platform API source.
- REMOTE_AND_SELF_PACED_ARCHITECTURE.md: closes the self-registration/curriculum-assignment/
  delivery-mode/enrollment-model gaps this doc previously flagged as Phase 5+ scope.
- ENGINEERING_SOP.md: new §15. LAB_ARCHITECTURE.md, PRODUCTION_VALIDATION_CHECKLIST.md,
  README.md, platform/README.md: cross-references and a Phase 5B production-validation section.

docs/PRODUCTION_VALIDATION_CHECKLIST.md

@@ -67,5 +67,20 @@ can prove in advance.
 - [ ] `docs/RUNBOOK_STUDENT_ACCOUNT_LIFECYCLE.md` and `docs/RUNBOOK_REMOTE_CLASSROOM.md`'s steps
       still make sense walked against the real admin portal
 
+## Self-paced enrollment & website integration (Phase 5B — see docs/PHASE5B_SELF_PACED_ENROLLMENT.md)
+- [ ] `INTEGRATION_SHARED_SECRET` is set to a real, unique-to-production secret (not the dev
+      value) and is stored only in the Platform API's own server-side environment — never
+      committed, never in any browser-reachable config
+- [ ] The website team's production integration key was delivered out of band (not plaintext
+      email/chat) and is confirmed working with one real, low-stakes test enrollment before go-live
+- [ ] A deliberate `POST /api/integrations/enrollments` call missing/with a wrong key returns a
+      clean `401` against the real production endpoint, not a 500 or an information leak
+- [ ] `GET /metrics`'s `rootnode_enrollments_total`/`rootnode_program_completions_total` labels
+      contain only status/delivery_mode/program_id values — confirm no student_id, email, or
+      external_reference ever appears in a real production scrape
+- [ ] `docs/RUNBOOK_SELF_PACED_ENROLLMENT.md`'s steps still make sense walked against the real
+      admin portal, and `docs/WEBSITE_ENROLLMENT_INTEGRATION.md` is what the website team
+      actually integrated against (update it if reality drifted during that integration)
+
 Only once every box above is checked against the real server does the deployment graduate from
 "infrastructure package built" to "actually in production."