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/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);