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>

docs/backend/README.md

@@ -0,0 +1,42 @@
+# Backend Contracts Index — Product Plan v3.1
+
+This directory is the complete set of wire contracts for building the backend behind [Product Plan v3.1](../PRODUCT-PLAN-v3.1-GAP-ANALYSIS.md). Each doc specifies entities, endpoints, and invariants only — never DB schema or service boundaries, which stay backend's own call.
+
+**Read order matches build order.** Every doc after Phase 1 depends on the ones before it (noted at the top of each). All Sprint 0.1 decisions referenced throughout were answered 2026-08-17 — see [PRODUCT-PLAN-v3.1-DELIVERY-PLAN.md](../PRODUCT-PLAN-v3.1-DELIVERY-PLAN.md) Sprint 0.1 for the full record.
+
+## Launch-gate phases (P0 — required before production)
+
+| Doc | Covers | Status |
+|---|---|---|
+| [PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md](PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md) | Money model, FX quote, price snapshot, server-authoritative checkout amount, payment state machine | Ready |
+| [PHASE-2-ORDERS-NOTIFICATIONS-CONTRACT.md](PHASE-2-ORDERS-NOTIFICATIONS-CONTRACT.md) | Canonical Order/OrderLine/Fulfillment (unified multi-seller), event bus, Notification Center | Ready |
+| [PHASE-3-CATALOG-OFFER-FULFILLMENT-CONTRACT.md](PHASE-3-CATALOG-OFFER-FULFILLMENT-CONTRACT.md) | Product/Offer split, inventory/reservations, publish-time executability | Ready |
+| [PHASE-4-CONNECTOR-FRAMEWORK-CONTRACT.md](PHASE-4-CONNECTOR-FRAMEWORK-CONTRACT.md) | Generic external-order connector framework (no fixed marketplace list) | Ready |
+
+## Post-launch-gate phases (P1/P2)
+
+| Doc | Covers | Status |
+|---|---|---|
+| [PHASE-5-SELLER-PORTAL-CONTRACT.md](PHASE-5-SELLER-PORTAL-CONTRACT.md) | Seller org/user/membership, seller-scoped order/fulfillment views | Ready |
+| [PHASE-6-CART-CHECKOUT-CONTRACT.md](PHASE-6-CART-CHECKOUT-CONTRACT.md) | Server-owned cart, checkout session | Ready |
+| [PHASE-7-PAYMENTS-RECONCILIATION-CONTRACT.md](PHASE-7-PAYMENTS-RECONCILIATION-CONTRACT.md) | Refunds, reconciliation, settlements | Ready |
+| [PHASE-8-IDENTITY-MESSAGING-CONTRACT.md](PHASE-8-IDENTITY-MESSAGING-CONTRACT.md) | Customer identity, VK ID (built first), OTP, MAX/Telegram bots, Notification Orchestrator | Ready |
+| [PHASE-9-TENANT-REGISTRY-DOMAINS-CONTRACT.md](PHASE-9-TENANT-REGISTRY-DOMAINS-CONTRACT.md) | Marketplace registry, Hostinger DNS automation, publish/revision model | Ready |
+| [PHASE-10-CONTENT-MODULES-CONTRACT.md](PHASE-10-CONTENT-MODULES-CONTRACT.md) | Gorbushka-class mall/directory content entities | Ready, lowest priority |
+
+## Cross-cutting tracks
+
+| Doc | Covers | Status |
+|---|---|---|
+| [TRACK-A-ANALYTICS-CONTRACT.md](TRACK-A-ANALYTICS-CONTRACT.md) | Event pipeline, funnel, operational/quality metrics, synthetic-traffic separation | Ready — start alongside Phase 1, longest lead time |
+| [TRACK-S-SECURITY-RBAC-CONTRACT.md](TRACK-S-SECURITY-RBAC-CONTRACT.md) | 17 roles/3 scopes, enforcement, audit log, secrets, rate limiting, step-up auth | Ready — gates the launch |
+
+## What is deliberately not in this directory
+
+- **API namespace migration** — Sprint 0.1 decision: new endpoints only use `/api/v2/...` etc; legacy endpoints (`/cart`, `/orders`, `/items`) are not being migrated as part of this contract set. See `BACKEND-API-REFERENCE.md` for the current live surface.
+- **Per-connector adapters** (Ozon, Wildberries, etc.) — Sprint 0.1 decision: no fixed list. [Phase 4](PHASE-4-CONNECTOR-FRAMEWORK-CONTRACT.md) §8 is the onboarding runbook; each partner's adapter is written when that partner is actually onboarded.
+- **Additional payment providers** (wallets, BNPL) — open business decision, not yet made. [Phase 7](PHASE-7-PAYMENTS-RECONCILIATION-CONTRACT.md) §4.
+
+## One open item across all of these
+
+**Backend ownership is still unanswered** (Sprint 0.1). Every contract above is ready to hand to whoever builds it — that person/team just hasn't been named yet.