feat: Phase 1 pricing core - Money/FxQuote/PriceSnapshot, mock-gateway backed

First frontend build against docs/backend/PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md:
- Money type (amountMinor + currency, no float math) with add/subtract/
  multiply helpers respecting per-currency minor-unit decimals.
- FxQuote model + FX_QUOTE_GATEWAY token, mirroring the DI-seam pattern
  already used for the 9 admin domains. FxQuoteLocalGateway derives a
  quote from the existing CurrencyRatesService so the shape is real even
  before a backend rate source exists (Sprint 0.1: FX is ours in-house).
  source: 'local-mock' is explicit and distinct from the eventual real
  backend's 'internal' - swapping the token when the real endpoint ships
  requires zero caller changes.
- PriceSnapshot/CheckoutLine/CheckoutResult models per contract §4-5.

Scope note: this does NOT yet rewire the live cart/checkout payment flow
(pages/cart/cart.component.ts) onto this module - that flow handles real
money against a live payment provider, and rewiring it deserves its own
focused pass with explicit verification, not a bundled mega-change. The
core module is ready for that pass.

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

src/app/core/pricing/models/fx-quote.model.ts

@@ -0,0 +1,14 @@
+/** Per docs/backend/PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md §3. */
+export interface FxQuote {
+  quoteId: string;
+  base: string;
+  quote: string;
+  rate: number;
+  source: string;
+  observedAt: string;
+  expiresAt: string;
+}
+
+export function isFxQuoteExpired(fxQuote: FxQuote, now: Date = new Date()): boolean {
+  return now.getTime() >= new Date(fxQuote.expiresAt).getTime();
+}

src/app/core/pricing/models/money.model.ts

@@ -0,0 +1,47 @@
+/** Minor-unit money, per docs/backend/PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md §2. No float for money math. */
+export interface Money {
+  amountMinor: number;
+  currency: string;
+}
+
+const MINOR_UNIT_DECIMALS: Record<string, number> = {
+  RUB: 2,
+  USD: 2,
+  EUR: 2,
+  AMD: 2,
+};
+
+export function decimalsFor(currency: string): number {
+  return MINOR_UNIT_DECIMALS[currency] ?? 2;
+}
+
+export function toMajor(money: Money): number {
+  return money.amountMinor / Math.pow(10, decimalsFor(money.currency));
+}
+
+export function fromMajor(amount: number, currency: string): Money {
+  const decimals = decimalsFor(currency);
+  return { amountMinor: Math.round(amount * Math.pow(10, decimals)), currency };
+}
+
+export function addMoney(a: Money, b: Money): Money {
+  if (a.currency !== b.currency) {
+    throw new Error(`Cannot add Money of different currencies: ${a.currency} vs ${b.currency}`);
+  }
+  return { amountMinor: a.amountMinor + b.amountMinor, currency: a.currency };
+}
+
+export function subtractMoney(a: Money, b: Money): Money {
+  if (a.currency !== b.currency) {
+    throw new Error(`Cannot subtract Money of different currencies: ${a.currency} vs ${b.currency}`);
+  }
+  return { amountMinor: a.amountMinor - b.amountMinor, currency: a.currency };
+}
+
+export function multiplyMoney(money: Money, factor: number): Money {
+  return { amountMinor: Math.round(money.amountMinor * factor), currency: money.currency };
+}
+
+export function zeroMoney(currency: string): Money {
+  return { amountMinor: 0, currency };
+}

src/app/core/pricing/models/price-snapshot.model.ts

@@ -0,0 +1,30 @@
+import { Money } from './money.model';
+
+/** Per docs/backend/PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md §4. Immutable once created. */
+export interface PriceSnapshot {
+  id: string;
+  offerId: string;
+  amount: Money;
+  displayAmount: Money;
+  fxQuoteId: string | null;
+  capturedAt: string;
+}
+
+export interface CheckoutLine {
+  offerId: string;
+  qty: number;
+  unitPrice: Money;
+  lineTotal: Money;
+  priceSnapshotId: string;
+}
+
+export interface CheckoutResult {
+  checkoutSessionId: string;
+  lines: CheckoutLine[];
+  subtotal: Money;
+  discount: Money;
+  delivery: Money;
+  total: Money;
+  fxQuoteId: string | null;
+  expiresAt: string;
+}

src/app/core/pricing/services/fx-quote-gateway.interface.ts

@@ -0,0 +1,7 @@
+import { Observable } from 'rxjs';
+import { FxQuote } from '../models/fx-quote.model';
+
+/** Per docs/backend/PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md §3.1. */
+export interface FxQuoteGateway {
+  getQuote(base: string, quote: string): Observable<FxQuote>;
+}

src/app/core/pricing/services/fx-quote-gateway.token.ts

@@ -0,0 +1,9 @@
+import { InjectionToken, inject } from '@angular/core';
+import { FxQuoteGateway } from './fx-quote-gateway.interface';
+import { FxQuoteLocalGateway } from './fx-quote-local.gateway';
+
+/** Swap point for the real FX backend from docs/backend/PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md §3.1. */
+export const FX_QUOTE_GATEWAY = new InjectionToken<FxQuoteGateway>('FX_QUOTE_GATEWAY', {
+  providedIn: 'root',
+  factory: () => inject(FxQuoteLocalGateway),
+});

src/app/core/pricing/services/fx-quote-local.gateway.ts

@@ -0,0 +1,32 @@
+import { Injectable, inject } from '@angular/core';
+import { Observable, of } from 'rxjs';
+import { FxQuote } from '../models/fx-quote.model';
+import { FxQuoteGateway } from './fx-quote-gateway.interface';
+import { CurrencyRatesService } from '../../../services/currency-rates.service';
+
+const QUOTE_TTL_MS = 5 * 60 * 1000;
+
+/**
+ * Mock FX source until a real backend rate service exists (Sprint 0.1:
+ * FX is computed in-house, "internal" is the normal source value, not just
+ * a fallback). Derives a quote from CurrencyRatesService's existing
+ * admin-editable rates so the shape is real even though the source isn't.
+ */
+@Injectable({ providedIn: 'root' })
+export class FxQuoteLocalGateway implements FxQuoteGateway {
+  private readonly currencyRates = inject(CurrencyRatesService);
+
+  getQuote(base: string, quote: string): Observable<FxQuote> {
+    const rate = this.currencyRates.convert(1, base, quote);
+    const now = new Date();
+    return of({
+      quoteId: `fxq_local_${base}_${quote}_${now.getTime()}`,
+      base,
+      quote,
+      rate,
+      source: 'local-mock',
+      observedAt: now.toISOString(),
+      expiresAt: new Date(now.getTime() + QUOTE_TTL_MS).toISOString(),
+    });
+  }
+}