docs: Track A/S contracts + backend index tying the full v3.1 contract set together

- Track A: analytics event pipeline (traffic/catalog/commerce/operational/
  quality events), synthetic-traffic separation enforced server-side by
  environment/token, never a client-settable flag.
- Track S: 17-role/3-scope RBAC enforcement, audit log, secrets, rate
  limiting, step-up auth - closes this session's most serious finding
  (admin role model is decorative today, any authenticated admin has
  full access regardless of assigned role).
- docs/backend/README.md: index of all 12 contract docs (Phases 1-10 +
  2 tracks) in build order, plus what's deliberately excluded (namespace
  migration, per-connector adapters, extra payment providers) and the
  one thing still genuinely open across all of them - backend ownership.
- Cross-linked from BACKEND-API-REFERENCE.md and the delivery plan so the
  index is discoverable from either entry point.

This closes out documentation for every phase/track in
PRODUCT-PLAN-v3.1-DELIVERY-PLAN.md that doesn't require a further business
decision. Nothing left undocumented on our side pending only implementation
and backend-ownership assignment.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

BACKEND-API-REFERENCE.md

@@ -2,6 +2,8 @@
 
 One document, everyone reads it: product, backend, frontend, QA. It answers three questions for every domain — **what does the frontend already call**, **what shape does it send/expect**, and **is it real or mocked today**. Generated from the actual Angular frontend source (this repo has no backend code — it is a pure client consuming an external API), cross-checked against the frontend's own tolerant adapters, not aspirational.
 
+**For what doesn't exist yet:** this doc describes the live surface only. The full set of forward-looking wire contracts for Product Plan v3.1 (money/FX, orders, catalog/offer split, connectors, seller portal, identity, tenant registry, RBAC, analytics — 10 phases + 2 tracks) lives in [docs/backend/](docs/backend/README.md).
+
 Maturity tags used throughout:
 
 | Tag | Meaning |