docs: record Sprint 0.1 decisions across delivery plan, gap analysis, backend contract

User answered 8 of 9 Sprint 0.1 blocking decisions (2026-08-17); backend
ownership stays open pending a clearer re-ask. Recorded and propagated:

- Payment chain unfrozen -> BACKEND-API-REFERENCE.md §7 and the Phase 1
  contract doc's status banner both updated; Phase 1/6/7 unblocked.
- No fixed external-marketplace list -> Phase 4's connector framework
  respecified as config-driven/generic; Sprint 4.2 retired as "per named
  marketplace," replaced with a generic onboarding runbook.
- FX rate source: ours, in-house, as the default (not just a fallback) ->
  Phase 1 contract's `source` field can read "internal" as the normal case.
- VK ID before OTP -> Phase 8 sprints resequenced (VK ID now 8.2, OTP 8.3).
- Multi-seller orders: unified -> Phase 3.3, Phase 5.2, and Z16 updated to
  the resolved model (one Order, per-seller Fulfillment groups).
- "Fixed 5-second payment" claim: confirmed non-issue, PAYMENT_POLL_INTERVAL_MS
  is already 5000 (real polling cadence, not an artificial delay).
- API namespace: new endpoints only (/api/v2/...), no forced migration of
  legacy endpoints.
- Document version: v3.1 is canonical.

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

BACKEND-API-REFERENCE.md

@@ -371,7 +371,7 @@ WebSessionID: 3f1c2a0e-…
 { "qrId": "QR-77f0", "nspkurl": "https://qr.nspk.ru/AD10…", "status": "created", "qrExpirationDate": "2026-07-26T04:10:00Z" }
 ```
 
-**Payments are frozen** — this call chain is explicitly out of scope for changes; document only, don't modify.
+**Payments were frozen; unfrozen 2026-08-17** (Sprint 0.1 decision, see `docs/PRODUCT-PLAN-v3.1-DELIVERY-PLAN.md`). This call chain is now in scope for the Phase 1 rework specified in `docs/backend/PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md` — the server-authoritative-amount contract there replaces the client-trusted `amount`/`price` fields described below.
 
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