feat: Phase 3 frontend - Offer/InventoryRecord core, mock-gateway backed

core/offers module against docs/backend/PHASE-3-CATALOG-OFFER-FULFILLMENT-
CONTRACT.md §2-3, §7. OfferLocalGateway derives one Offer per existing
AdminProduct (sellerId defaults to 'marketplace-owned' when absent - same
convention AdminProduct.sellerId already documents) so the shape is real
without touching the live admin Products domain.

Scope note: this is the core swappable seam only (model + gateway + token),
same judgment as Phase 1's pricing core - deferred is the actual Product/
Offer split UI (multi-seller product page, admin lookup-by-SKU screen),
which is the single largest structural change in the whole programme per
the delivery plan and needs its own dedicated pass against a real backend,
not a bundled mock-data rewire of the working Products admin surface.

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

src/app/core/offers/models/offer.model.ts

@@ -0,0 +1,30 @@
+import { Money } from '../../pricing/models/money.model';
+
+/** Per docs/backend/PHASE-3-CATALOG-OFFER-FULFILLMENT-CONTRACT.md §2-3. */
+export interface Offer {
+  id: string;
+  marketplaceId: string;
+  sellerId: string;
+  variantId: string;
+  sellerSku: string;
+  price: Money;
+  stockPolicy: 'track' | 'no_track' | 'preorder';
+  status: 'draft' | 'moderation' | 'published' | 'paused' | 'archived';
+  publishedAt?: string;
+  executabilityChecked: boolean;
+}
+
+export interface InventoryRecord {
+  offerId: string;
+  available: number;
+  reserved: number;
+  sold: number;
+  warehouse?: string;
+  source: 'manual' | 'feed_sync' | 'connector';
+}
+
+export interface OfferLookupQuery {
+  sku?: string;
+  sellerSku?: string;
+  externalId?: string;
+}

src/app/core/offers/services/offer-gateway.interface.ts

@@ -0,0 +1,10 @@
+import { Observable } from 'rxjs';
+import { InventoryRecord, Offer, OfferLookupQuery } from '../models/offer.model';
+
+/** Per docs/backend/PHASE-3-CATALOG-OFFER-FULFILLMENT-CONTRACT.md §7. */
+export interface OfferGateway {
+  loadOffers(sellerId?: string): Observable<Offer[]>;
+  lookup(query: OfferLookupQuery): Observable<Offer[]>;
+  loadInventory(offerId: string): Observable<InventoryRecord | null>;
+  publish(offerId: string): Observable<{ ok: true } | { ok: false; errors: string[] }>;
+}

src/app/core/offers/services/offer-gateway.token.ts

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+import { InjectionToken, inject } from '@angular/core';
+import { OfferGateway } from './offer-gateway.interface';
+import { OfferLocalGateway } from './offer-local.gateway';
+
+/** Swap point for docs/backend/PHASE-3-CATALOG-OFFER-FULFILLMENT-CONTRACT.md §7. */
+export const OFFER_GATEWAY = new InjectionToken<OfferGateway>('OFFER_GATEWAY', {
+  providedIn: 'root',
+  factory: () => inject(OfferLocalGateway),
+});

src/app/core/offers/services/offer-local.gateway.ts

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+import { Injectable, inject } from '@angular/core';
+import { Observable, map, of } from 'rxjs';
+import { InventoryRecord, Offer, OfferLookupQuery } from '../models/offer.model';
+import { OfferGateway } from './offer-gateway.interface';
+import { ADMIN_PRODUCTS_GATEWAY } from '../../../features/admin/products/services/admin-products-gateway.token';
+import { fromMajor } from '../../pricing/models/money.model';
+
+/**
+ * Derives Offers from the existing (mock) Products gateway - one offer per
+ * product, sellerId defaulted to 'marketplace-owned' when absent, exactly
+ * matching AdminProduct.sellerId's existing "absent means marketplace-owned"
+ * convention. Real Offer/Product split (docs/backend/
+ * PHASE-3-CATALOG-OFFER-FULFILLMENT-CONTRACT.md) replaces this once a real
+ * backend exists - swap OFFER_GATEWAY, no caller changes needed.
+ */
+@Injectable({ providedIn: 'root' })
+export class OfferLocalGateway implements OfferGateway {
+  private readonly productsGateway = inject(ADMIN_PRODUCTS_GATEWAY);
+
+  loadOffers(sellerId?: string): Observable<Offer[]> {
+    return this.productsGateway.loadProducts({ search: '', page: 1, pageSize: 100 } as any).pipe(
+      map((result: any) => (result.items ?? [])
+        .map((p: any): Offer => this.toOffer(p))
+        .filter((o: Offer) => !sellerId || o.sellerId === sellerId))
+    );
+  }
+
+  lookup(query: OfferLookupQuery): Observable<Offer[]> {
+    return this.loadOffers().pipe(
+      map(offers => offers.filter(o =>
+        (!query.sku || o.sellerSku === query.sku) &&
+        (!query.sellerSku || o.sellerSku === query.sellerSku)
+      ))
+    );
+  }
+
+  loadInventory(offerId: string): Observable<InventoryRecord | null> {
+    return of({ offerId, available: 42, reserved: 0, sold: 0, source: 'manual' });
+  }
+
+  publish(_offerId: string): Observable<{ ok: true } | { ok: false; errors: string[] }> {
+    return of({ ok: true });
+  }
+
+  private toOffer(product: any): Offer {
+    return {
+      id: `offer_${product.id}`,
+      marketplaceId: 'default',
+      sellerId: product.sellerId ?? 'marketplace-owned',
+      variantId: product.id,
+      sellerSku: product.sku,
+      price: fromMajor(product.price ?? 0, product.currency ?? 'RUB'),
+      stockPolicy: 'track',
+      status: product.visible ? 'published' : 'draft',
+      executabilityChecked: !!product.visible,
+    };
+  }
+}