docs: partner provisioning API contract, routing context, Track P

A partner integration request landed for programmatic merchant-hierarchy
management (Company/Project/Store/PaymentPoint). Built the answer generically:
partner-specific behaviour is a PartnerProfile config row, and no partner name
appears in any entity, field, endpoint or status value.

New:
- docs/backend/PARTNER-PROVISIONING-API-CONTRACT.md - hierarchy, idempotency,
  node-scoped public-key credentials, TEST/LIVE partition, routing context
- docs/context/adrs/ADR-0003-generic-partner-provisioning-api.md

Amended, because the schema impact must land before Phase 1 is implemented:
- Phase 1 gains RoutingContext on CheckoutSession/PaymentIntent/Payment,
  frozen at checkout-session creation and immutable after
- Phase 7 gains routing on Refund/ReconciliationRecord, plus the rule that
  seller settlement splits happen after routing, never as a hierarchy level
- Phase 9 gains Company/Project above Marketplace and PaymentPoint below it,
  with a backfill sequence for existing marketplaces
- Track S gains partner credentials: public key only, node-scoped authority,
  rotation with overlap, immediate revoke, audit coverage

Also: Track P (P1-P10) in the delivery plan, and backend ownership closed as
answered across the contract set.

Card payment was checked, not added - qr and card both already ship in
cart.component.ts with separate create paths and status pollers.

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

docs/backend/README.md

@@ -32,6 +32,7 @@ This directory is the complete set of wire contracts for building the backend be
 |---|---|---|
 | [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 |
+| [PARTNER-PROVISIONING-API-CONTRACT.md](PARTNER-PROVISIONING-API-CONTRACT.md) | Inbound partner API: merchant hierarchy provisioning, idempotency, public-key credentials, payment routing context | Draft — mapping decided, needs Company/Project entities |
 
 ## What is deliberately not in this directory
 
@@ -41,4 +42,4 @@ This directory is the complete set of wire contracts for building the backend be
 
 ## 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.
+**Backend ownership — answered 2026-08-18.** A separate backend developer implements against these contracts. This repository's team owns the frontend and owns *this contract set* — the docs here are the handoff surface between the two, so a change to any contract is a change both sides must see. Keep them current; they are not a one-time deliverable.