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/BACKEND-HANDOFF.md

@@ -29,6 +29,7 @@ Constraints already fixed by the frontend design (see the platform-vision facts
 4. [TRACK-S-SECURITY-RBAC-CONTRACT.md](TRACK-S-SECURITY-RBAC-CONTRACT.md) — **gates the launch.** Today the admin role model is decorative: nothing server-side enforces any permission. §8 covers per-marketplace bootstrap admin accounts and self-service sub-admin management.
 5. [TRACK-A-ANALYTICS-CONTRACT.md](TRACK-A-ANALYTICS-CONTRACT.md) — longest lead time, start it in parallel with Phase 1.
 6. Phases 5→10 — post-launch-gate.
+7. [PARTNER-PROVISIONING-API-CONTRACT.md](PARTNER-PROVISIONING-API-CONTRACT.md) — the inbound partner API. Read it **before implementing Phase 1**, not after: it adds `RoutingContext` to `CheckoutSession`/`PaymentIntent`/`Payment` ([Phase 1 §6.5](PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md)) and two levels above `Marketplace` ([Phase 9 §1](PHASE-9-TENANT-REGISTRY-DOMAINS-CONTRACT.md)). Building the partner API itself can wait; carrying its routing dimension in the payments tables cannot.
 
 [../../BACKEND-API-REFERENCE.md](../../BACKEND-API-REFERENCE.md) documents the *current* live API surface (legacy endpoints, error envelope, mock-only areas). New endpoints use `/api/v2/...` namespaces; legacy endpoints are not being migrated.
 
@@ -90,6 +91,7 @@ Angular 22, Node 20+. nginx serves `/srv/marketplaces/current/frontend`, so depl
 ## 7. Known open decisions
 
 - Registry reachability for CI (reverse proxy + TLS, or a different registry entirely).
-- Backend ownership was still unnamed as of Sprint 0.1.
+- ~~Backend ownership.~~ Answered 2026-08-18: implemented by a separate backend developer against this contract set.
 - Additional payment providers (wallets, BNPL) — [Phase 7 §4](PHASE-7-PAYMENTS-RECONCILIATION-CONTRACT.md).
 - Per-connector marketplace adapters — written per partner at onboarding, [Phase 4 §8](PHASE-4-CONNECTOR-FRAMEWORK-CONTRACT.md).
+- Backfill of `Company`/`Project`/`PaymentPoint` for existing marketplaces — sequence specified in [Phase 9 §1.2](PHASE-9-TENANT-REGISTRY-DOMAINS-CONTRACT.md), not yet scheduled.