refactor: rename storefront CategoryApiModel; correct stale auth-error doc; add Phase 1 backend contract

- models/category.model.ts: Category -> CategoryApiModel, disambiguated
  from core/categories/models/category-domain.model.ts's Category (admin
  domain shape). Removes a dead unused import in item.utils.ts along the
  way. Only live consumer was services/api.service.ts, updated in place.
- BACKEND-API-REFERENCE.md §5: corrected two rows documenting the
  TOKEN_EXPIRED/INVALID_SIGNATURE auth-error bug as still open - the fix
  (reading error.error.code before falling back to HTTP status) is
  already in auth.service.ts. Doc was stale, not the code.
- Sprint 0.2 audit: AdminRole duplication and the
  PRODUCT_DATA_PROVIDER/CATEGORY_REPOSITORY dead mock branches were
  already resolved in a prior pass - verified, no code change needed.
- docs/backend/PHASE-1-MONEY-FX-PAYMENTS-CONTRACT.md: new wire contract
  for Money/FxQuote/PriceSnapshot/payment state machine, so backend can
  start Phase 1 the moment the frozen payment chain is unblocked.

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

src/app/models/category.model.ts

@@ -1,6 +1,11 @@
 import { ItemName } from './item.model';
 
-export interface Category {
+/**
+ * Storefront-side raw category shape (item.service.ts/api.service.ts responses).
+ * Unrelated to core/categories/models/category-domain.model.ts's Category, which is
+ * the normalized admin-backoffice domain shape produced by CategoryMapper.
+ */
+export interface CategoryApiModel {
   categoryID: number;
   name: string;
   parentID: number;

src/app/services/api.service.ts

@@ -2,7 +2,7 @@ import { Injectable } from '@angular/core';
 import { HttpClient, HttpHeaders, HttpParams } from '@angular/common/http';
 import { Observable, timer } from 'rxjs';
 import { map, retry } from 'rxjs/operators';
-import { Category, DeliveryOption, Item, Subcategory } from '../models';
+import { CategoryApiModel, DeliveryOption, Item, Subcategory } from '../models';
 import { normalizeDeliveryOption, normalizeOptionalNumber } from '../utils/normalization.utils';
 import { environment } from '../../environments/environment';
 import { ApiConfigService } from '../core/config/api-config.service';
@@ -199,7 +199,7 @@ export class ApiService {
       || (subcategory.subcategories?.length ?? 0) > 0;
   }
 
-  private isDisplayableCategory(category: Category): boolean {
+  private isDisplayableCategory(category: CategoryApiModel): boolean {
     return category.visible !== false;
   }
 
@@ -462,8 +462,8 @@ export class ApiService {
    * Normalize a category from the API response — supports both
    * the flat legacy format and nested backOffice format.
    */
-  private normalizeCategory(raw: any): Category {
-    const cat: Category = { ...raw };
+  private normalizeCategory(raw: any): CategoryApiModel {
+    const cat: CategoryApiModel = { ...raw };
 
     if (raw.id != null && raw.categoryID == null) {
       cat.id = String(raw.id);
@@ -522,7 +522,7 @@ export class ApiService {
     return cat;
   }
 
-  private normalizeCategories(cats: any[] | null | undefined): Category[] {
+  private normalizeCategories(cats: any[] | null | undefined): CategoryApiModel[] {
     if (!cats || !Array.isArray(cats)) return [];
     return cats
       .map(c => this.normalizeCategory(c))
@@ -535,7 +535,7 @@ export class ApiService {
     return this.http.get<{ message: string }>(`${this.baseUrl}/ping`);
   }
 
-  getCategories(): Observable<Category[]> {
+  getCategories(): Observable<CategoryApiModel[]> {
     return this.http.get<any[]>(`${this.baseUrl}/category`)
       .pipe(retry(this.retryConfig), map(cats => this.normalizeCategories(cats)));
   }

src/app/utils/item.utils.ts

@@ -1,5 +1,4 @@
 import { Item } from '../models';
-import { Category } from '../models/category.model';
 
 export function getDiscountedPrice(item: Item): number {
   return item.price * (1 - (item.discount || 0) / 100);