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/PRODUCT-PLAN-v3.1-DELIVERY-PLAN.md

@@ -1,6 +1,6 @@
 # Product Plan v3.1 — Delivery Plan (Phases → Sprints → Todos)
 
-Companion to [PRODUCT-PLAN-v3.1-GAP-ANALYSIS.md](PRODUCT-PLAN-v3.1-GAP-ANALYSIS.md). Every gap identified there is assigned here exactly once.
+Companion to [PRODUCT-PLAN-v3.1-GAP-ANALYSIS.md](PRODUCT-PLAN-v3.1-GAP-ANALYSIS.md). Every gap identified there is assigned here exactly once. Wire contracts for every `[BE]`/`[BOTH]` phase and track below are written up in [docs/backend/](backend/README.md) — hand that directory to whoever builds the backend.
 
 **No calendar dates.** The plan itself (§12) refuses invented dates and fixes *sequence + exit criteria* instead. This document does the same. Sprints are ordered units of work, not two-week promises. Sizes are relative: **S** / **M** / **L** / **XL**.