docs: assemble BACKEND_INTEGRATION.md, single canonical backend spec

4349 lines, 9 numbered sections per the Backend Finalization Sprint
spec: Bootstrap, Endpoint Framework, CRUD Contracts (~102 endpoints
across 20 domains), Authentication (spliced from AUTHENTICATION.md),
Security, Error Model (spliced from ERROR_CONTRACT.md), Uploads, Real
Backend Implementation Guide, Backend Checklist (34 items).

Everything traced to docs/context/BACKEND-AUDIT.md and actual current
source - proposed (unverified) paths explicitly marked as such,
everything the frontend has no opinion on marked "Requires backend
decision" rather than invented.

Archived the three docs this supersedes (BACKEND_API.md, AUTH.md,
BACKEND_API_REMAINING_WORK.md) to docs/archive/ with pointers back to
this file. AUTHENTICATION.md, ERROR_CONTRACT.md, MAINTENANCE_MODE.md
kept in place as standalone companion references (their content is
also inlined/cross-referenced here). ADMIN.md left untouched - it's a
frontend admin-UI sprint doc, not a backend spec, no overlap.

Verified via repo-wide search: no other backend/API spec docs remain
outside archive/ and this canonical file.

docs/archive/AUTH.md

@@ -1,3 +1,5 @@
+> **ARCHIVED 2026-07-26.** Superseded by [`docs/BACKEND_INTEGRATION.md`](../BACKEND_INTEGRATION.md), the single canonical backend integration document. Kept for history only — do not implement against this file.
+
 # Admin Authentication — Ed25519 Foundation
 
 Status: **FRONTEND PREPARED, NOT LIVE.** Everything in this document describes

docs/archive/BACKEND_API.md

@@ -1,3 +1,5 @@
+> **ARCHIVED 2026-07-26.** Superseded by [`docs/BACKEND_INTEGRATION.md`](../BACKEND_INTEGRATION.md), the single canonical backend integration document. Kept for history only — do not implement against this file.
+
 # Backend Integration — Master Specification
 
 Status: canonical. This is the single authoritative backend/API contract for this Angular multi-tenant marketplace platform. It supersedes and replaces `docs/BACKEND.md`, `docs/BACKEND_API.md` (old), `docs/BACKEND-INTEGRATION-PROMPT.md`, `docs/BACKEND-DIFF-VS-MAIN.md`, `docs/architecture/backend/Backend-Platform-API-Spec.md`, and the API-contract portions of `docs/BOOTSTRAP.md`. Those files are deleted; this document is where all of that content now lives, verified against the current source tree on branch `B2B`.

docs/archive/BACKEND_API_REMAINING_WORK.md

@@ -1,3 +1,5 @@
+> **ARCHIVED 2026-07-26.** Superseded by [`docs/BACKEND_INTEGRATION.md`](../BACKEND_INTEGRATION.md), the single canonical backend integration document. Kept for history only — do not implement against this file.
+
 # Remaining backend work (everything except auth/session)
 
 Companion to the `API-CONTRACT.md` backend delivered separately (covers `GET /bootstrap`