docs: Backoffice readiness audit for future Seller Management

Audit only, no code changed - every fact gathered by reading current
facades/gateways/components on this branch, not assumed. Covers all
13 admin modules (Dashboard, Products, Categories, Orders, Customers,
Users, Analytics, Reviews/Moderation, Media, CMS, Builder, Settings,
Monitoring, Transactions).

Per module: answers the 3 readiness questions (does Marketplace Owner
see everything / would Seller see only their own / would Seller Staff
be limited), documents where a future scope would be injected (an
existing method/interface parameter to extend - no "if seller" checks
introduced anywhere), lists components that currently assume global
ownership, and classifies Ready / Needs scope / Needs permissions /
Needs API change.

Key findings:
- Only 3 of 13 gateways (Categories, Dashboard-metrics, Media) are
  DI-token-swappable today; everything else needs that seam added
  first, independent of seller scoping.
- Orders is the load-bearing blocker: Customers, Transactions, and
  half of Analytics all derive from its same unscoped full-fetch order
  list, and AdminOrderItem has no per-item seller attribution at all -
  the concrete gap behind Seller-Management.md's open Unified-vs-Split-
  Orders question.
- Users already carries an AdminUserScope/AdminRole concept (label-
  only today) - the natural future home for the Marketplace Owner/
  Seller/Seller Staff/Platform Admin role vocabulary.
- CMS/Static Pages and Builder/Project Editor are structurally not
  about data scoping at all (marketplace-wide content, single global
  config document respectively) - seller-level work there is new
  product surface, not an extension.
- No admin module anywhere does role-based hiding of buttons or data
  today - confirmed, not assumed.

Linked from docs/architecture/foundation/README.md alongside the
other Seller Management docs.

docs/architecture/foundation/README.md

@@ -54,6 +54,7 @@ It is a platform runtime that must support unlimited tenants from one Angular ap
 - [Seller-Management-Diagrams.md](Seller-Management-Diagrams.md) — hierarchy, bootstrap gate, type diagram
 - [Seller-Management-Domain-Models.md](Seller-Management-Domain-Models.md) — typed models, optional sellerId fields
 - [Seller-Management-UX-Review.md](Seller-Management-UX-Review.md) — UX/accessibility review of the Phase 1 UI
+- [Seller-Management-Backoffice-Readiness-Audit.md](Seller-Management-Backoffice-Readiness-Audit.md) — per-module scoping/permissions readiness audit
 
 ### Engineering Rule Documents