feat(platform): pagination, admission control, cohort roles, version endpoint, structured assessment editor

- Real server-side pagination (platform/api/src/pagination.js) on the audit
  log (was a flat LIMIT 200), student attempt history (was LIMIT 100), and
  assessment session history (was fully unbounded - the one genuine growth
  risk found). Frontend updated (AuditView, AssessmentsView, StudentsView).
- Admission control: resource-policy.yaml's new (disabled-by-default)
  admission_control.max_concurrent_labs, checked before touching Docker at
  all, returns a clean 503 CAPACITY_EXCEEDED instead of failing mid-provision.
  Relayed as a clean student-facing message, never a raw 500. New
  GET /capacity endpoint feeds the instructor dashboard.
- cohort_instructors.role_in_cohort (existed since Phase 2B, never read) is
  now enforced: teacher vs viewer, rbac.js's
  requireCohortWriteAccess/requireStudentCohortWriteAccess.
- GET /api/version - app version, deployed Git SHA, latest migration as a
  schema-version stand-in.
- Structured assessment authoring UI (AssessmentsView.jsx's PoolSpecEditor)
  replaces the raw-JSON pool_spec textarea, backed by a new
  GET /api/admin/scenario-catalog endpoint. Owner-only raw-JSON fallback
  remains for anything the structured editor doesn't cover.
- NEEDS_ATTENTION_FAIL_THRESHOLD/STALE_DAYS moved to env-var configuration.

Full regression suite (e2e-phase2a/2b/3a/3b, 67/67) re-run clean after
these changes; frontend production build confirmed clean.

infra/ansible/roles/platform/templates/platform.env.j2

@@ -16,3 +16,9 @@ INSTRUCTOR_SESSION_IDLE_HOURS=2
 
 COOKIE_SECURE=1
 TRUST_PROXY=1
+
+# Phase 4.1 §17 — instructor "needs attention" thresholds. Commented out (code defaults apply:
+# 3 failures, 7 stale days) — uncomment and override in group_vars/{{ env }}.yml only if a
+# cohort's real usage shows the defaults are wrong for it.
+# NEEDS_ATTENTION_FAIL_THRESHOLD=3
+# NEEDS_ATTENTION_STALE_DAYS=7