# Phase 1 Backend Contract — Money, FX, Price Snapshot, Payment State Machine

Companion to [PRODUCT-PLAN-v3.1-DELIVERY-PLAN.md](../PRODUCT-PLAN-v3.1-DELIVERY-PLAN.md) Phase 1 (Sprints 1.1–1.4) and [PRODUCT-PLAN-v3.1-GAP-ANALYSIS.md](../PRODUCT-PLAN-v3.1-GAP-ANALYSIS.md) §3.3/§3.5/§3.6.

**Status: unblocked (2026-08-17).** [BACKEND-API-REFERENCE.md §7](../../BACKEND-API-REFERENCE.md) previously marked the cart/payment call chain frozen. Per the delivery plan's Sprint 0.1 decision, the freeze is lifted — this contract can move to implementation once backend ownership (also Sprint 0.1, still open) is confirmed.

This doc is the frontend's ask, in the same style as `BACKEND-API-REFERENCE.md`. It does not prescribe backend implementation (DB schema, service boundaries) — only the wire contract and the invariants the frontend needs to hold.

---

## 1. Why this exists

Current behaviour (`services/currency-rates.service.ts`, `pages/cart/cart.component.ts`):

- Currency conversion rates are typed by an admin into Admin Settings and persisted to browser `localStorage`. They never update and drift from market.
- The amount charged is computed **client-side** and sent as `CartPaymentRequest.amount` to `POST /cart`. The backend currently trusts this number.
- No record exists anywhere of which FX rate produced a given displayed price, or when it was captured.

Result: bank/NSPK settlement totals don't reconcile against order counts, because nothing on the backend can reconstruct *why* a given amount was charged. This document's contract exists to close that gap — it is the same complaint as Product Plan v3.1 §3.3/§3.8, and our own [§12.7](../../BACKEND-API-REFERENCE.md) raised it first.

---

## 2. Money representation

All money fields in every new endpoint below use minor units, never float.

```ts
interface Money {
  amountMinor: number; // integer, no float. 4990 = 49.90 for a 2-decimal currency.
  currency: string;    // ISO 4217, e.g. "RUB" | "USD" | "EUR" | "AMD"
}
```

| Currency | Minor unit | Decimals |
|---|---|---|
| RUB | kopeck | 2 |
| USD | cent | 2 |
| EUR | cent | 2 |
| AMD | luma | 2 |

Rounding rule for any conversion: round half up to the currency's minor-unit precision, applied once, at the point of conversion — never re-rounded on redisplay.

---

## 3. FX Quote

### 3.1 Endpoint

```
GET /api/v2/pricing/fx-quote?base=RUB&quote=USD
```

```json
{
  "quoteId": "fxq_8a3f1c2a",
  "base": "RUB",
  "quote": "USD",
  "rate": 0.0108,
  "source": "rapira",
  "observedAt": "2026-08-20T09:14:00Z",
  "expiresAt": "2026-08-20T09:19:00Z"
}
```

| Field | Notes |
|---|---|
| `quoteId` | Opaque, referenced by every `PriceSnapshot` that used this quote. |
| `rate` | `1 base = rate * quote`. Float is acceptable here — it's a market rate, not a money amount. |
| `source` | Adapter name. Frontend never hardcodes a provider; treat as an opaque label for display in the backoffice reconciliation panel. |
| `expiresAt` | TTL, provider-configurable. Frontend must not use an expired quote to display or charge. |

### 3.2 Stale-quote policy

- If the frontend holds a quote past `expiresAt`, it must re-fetch before checkout can proceed.
- If the rate source is unavailable, the backend decides: **block** (`503 SERVICE_UNAVAILABLE` with `error.code: "FX_SOURCE_UNAVAILABLE"`) or serve a configured fallback quote explicitly marked `"source": "fallback"`. Which policy applies is a tenant setting, not a frontend choice — see delivery-plan Sprint 0.1 decision on FX source.
- Outlier detection (e.g. a quote >X% off the previous one) is a backend concern; the frontend has no opinion on the threshold, only on obeying `expiresAt`.

---

## 4. PriceSnapshot

Created once, at checkout, immutable afterward. This is what makes a total explainable months later.

```ts
interface PriceSnapshot {
  id: string;
  offerId: string;
  amount: Money;          // price in the offer's base currency
  displayAmount: Money;    // price in the currency the customer checked out in
  fxQuoteId: string | null; // null when displayAmount.currency === amount.currency
  capturedAt: string;      // ISO 8601
}
```

Rule: once a `PriceSnapshot` exists on an order line, it is never recalculated — not on rate update, not on currency-setting change, not on replay. An old order shows the price it was actually charged at.

---

## 5. Server-authoritative checkout amount

This is the contract change with the highest priority in Phase 1 — it removes the client-trusted `amount` field entirely.

### 5.1 Current (to be replaced)

```http
POST /cart
{ "amount": 4990, "currency": "RUB", "items": [{ "itemID": 101, "price": 4990, ... }], ... }
```

The backend trusts `amount` and each line's `price` as sent by the browser.

### 5.2 Target

```http
POST /api/v2/storefront/checkout
{
  "offers": [{ "offerId": "off_9a1", "qty": 2 }],
  "currency": "USD",
  "deliveryOptionId": "del_standard"
}
```

```json
{
  "checkoutSessionId": "chk_7f2e",
  "lines": [
    {
      "offerId": "off_9a1",
      "qty": 2,
      "unitPrice": { "amountMinor": 5390, "currency": "USD" },
      "lineTotal": { "amountMinor": 10780, "currency": "USD" },
      "priceSnapshotId": "snap_3b1c"
    }
  ],
  "subtotal": { "amountMinor": 10780, "currency": "USD" },
  "discount": { "amountMinor": 0, "currency": "USD" },
  "delivery": { "amountMinor": 500, "currency": "USD" },
  "total": { "amountMinor": 11280, "currency": "USD" },
  "fxQuoteId": "fxq_8a3f1c2a",
  "expiresAt": "2026-08-20T09:19:00Z"
}
```

**The frontend sends offer IDs and quantities. The backend computes every price, using the offer's live price and the current FX quote. No `amount` or `price` field is ever accepted from the client for anything that affects the charge.**

`POST /api/v2/storefront/payments/intents` then references `checkoutSessionId` only — the amount charged is read server-side from the checkout session, never re-sent by the client.

### 5.3 Total formula (must be reconstructable, per line)

```
order.total = sum(line.unitPrice * line.qty)
              - discounts
              + delivery
              + taxes/fees (if applicable)
```

Backoffice must be able to render this formula, with the FX quote used, for any order — this is what Product Plan §7.2 asks for and what a bank reconciliation needs.

---

## 6. Payment state machine

### 6.1 States

```
PaymentIntent: created -> pending -> authorized/paid -> failed/cancelled
Payment:       received -> confirmed -> captured/settled -> refunded/partially_refunded
Order:         pending_payment -> paid -> processing -> fulfilled/completed
```

### 6.2 Required fields per transition

```ts
interface PaymentEvent {
  id: string;
  paymentIntentId: string;
  fromState: string;
  toState: string;
  providerEventId: string;   // idempotency key from the provider
  providerTimestamp: string; // when the provider says it happened
  receivedAt: string;        // when our webhook received it
  processedAt: string;       // when our system finished processing it
}
```

No fixed delays anywhere in this chain. The frontend already complies with this (polls real provider status via `/qr/dynamic/{partnerId}/{qrId}` and `/card/{partnerId}/{orderId}` on an interval bounded by QR TTL) — this section documents the backend side of the same principle.

### 6.3 Webhook contract

```
POST /api/providers/v1/payments/{provider}/webhook
```

- Signature verification is mandatory; reject unsigned/invalid-signature payloads with `401`, do not silently accept.
- Idempotency key = `provider + providerEventId`. A repeated delivery of the same event must be a no-op — same `PaymentEvent` row, no second order, no second notification.
- On success, emit `payment.confirmed` / `payment.failed` onto the platform event bus (Phase 2) so Order creation is driven by the event, not by the webhook handler doing double duty.

### 6.4 Idempotent order creation

```
POST /api/admin/v2/orders  (internal, from the payment-confirmation handler)
Idempotency-Key: <checkoutSessionId>
```

A retried call with the same `checkoutSessionId` must return the existing order, not create a second one. This is the mechanism that makes "double-click doesn't create two orders" true regardless of frontend debouncing.

---

## 7. What the frontend will stop doing once this ships

- Delete `CurrencyRatesService`'s `localStorage`-persisted admin-typed rates and hardcoded `DEFAULT_RATES` fallback (`USD: 0.011`, `AMD: 4.3`).
- Delete the Admin Settings currency-rate editor UI.
- Stop sending `amount` / `price` in any checkout-related request.
- Replace client-side float conversion (`CurrencyRatesService.convert()`) with server-supplied `Money` values everywhere a price is displayed.

## 8. What the frontend will start doing

- Fetch `GET /api/v2/pricing/fx-quote` on currency switch; block checkout if the held quote has expired.
- Render the backoffice "total formula" panel (lines × qty − discounts + delivery + fees, FX quote used) once §5.2 and the admin Orders API exist (Phase 2).
- Surface `FX_SOURCE_UNAVAILABLE` and `error.code`-driven stale-quote UI per the error envelope in `BACKEND-API-REFERENCE.md §5`.

---

## 9. Resolved / open questions (Sprint 0.1, 2026-08-17)

1. **Payment chain freeze — lifted.** §5 can proceed.
2. **FX rate source/provider — ours, in-house, as the default (not just a fallback).** No external provider committed. Backend computes and serves the quote itself; the `source` field in §3.1 can legitimately read `"internal"` as the normal case. Revisit if an external provider is chosen later — the contract shape doesn't need to change, only the value of `source`.
3. **Backend-converted prices vs. frontend-requested display currency — still open, needs confirmation before implementation.** This doc's §5.2 models the frontend sending a target `currency` and the backend returning the converted total. Confirm this is the intended flow before backend implementation starts.
4. **Backend ownership — still open.** This contract is ready regardless of who builds against it, but implementation can't be scheduled until this is answered.
