docs: final documentation consolidation - one canonical doc set

Audited every *.md in docs/ and root. Merged five overlapping backend
docs (BACKEND_INTEGRATION.md + AUTHENTICATION.md + ERROR_CONTRACT.md +
MAINTENANCE_MODE.md + the already-archived BACKEND_API.md/
BACKEND_API_REMAINING_WORK.md) into one canonical docs/BACKEND.md
(4775 lines, 10 numbered sections) - deleted the four standalone
files outright now that their content is fully inlined.

Archived (not deleted - real historical value): ADMIN.md (Sprint
19-28 build log, sprint-report-shaped, not a living reference) and
FRONTEND-ROADMAP.md (despite its name, a shipped-history changelog
with detail no other doc has - not a forward roadmap, so keeping it
in root alongside NEXT_PHASE.md was exactly the "10 roadmaps"
confusion being cleaned up).

Deleted outright (zero value): SPRINTS.md - a leftover copy-pasted
sprint-kickoff prompt saved as a file, not documentation.

Rewrote docs/PROJECT_STATUS.md with completion-percentage estimates
per area (frontend/backend/UI/admin/storefront) and an explicit
first-customer-readiness call. Rewrote docs/NEXT_PHASE.md to the
strict 5-phase structure (backend integration -> production testing
-> performance -> monitoring -> v2 ideas), pointing to PRODUCT_BACKLOG
.md/FUTURE_FEATURES.md for phase 5 detail instead of duplicating it.

Rewrote root README.md - was stale (referenced deleted pages/info,
pages/legal folders from a prior RC pass), now covers architecture,
frontend/backend status, how to run, mock<->API switch mechanism
(useMockData in environment.ts), current folder structure, and a
documentation map.

Updated docs/PROJECT_INDEX.md (the stated entry point) to link only
the surviving doc set - every remaining document is reachable from it.

Fixed every broken/stale cross-reference to the deleted/renamed
backend docs across ARCHITECTURE.md, EDITOR.md, FRONTEND.md,
PROJECT-STRUCTURE.md, StaticPages.md, KNOWN-ISSUES.md (10 individual
link fixes, verified by repo-wide grep before and after). Left
CHANGELOG.md's two historical entries untouched - changelogs are
append-only history, not live navigation, editing past entries would
misrepresent what was true at the time.

Not touched (explicitly out of scope): docs/architecture/foundation/**
(enforced ADRs/governance, permanent not sprint-shaped),
docs/context/** (Barry Cache infrastructure, "do not edit by hand"
per CLAUDE.md), .claude/worktrees/** (separate git worktrees
containing an unrelated project's docs, not this repo's documentation).

docs/ root: 22 files -> 16. Plus 5 in docs/archive/ (was 3).

docs/FRONTEND.md

@@ -25,7 +25,7 @@ src/app/
 - Storefront: `/`, `/catalog`, `/catalog/:id`, `/product/:id` (legacy `/item/:id` and `/category/:id[/items]` redirect for compatibility).
 - Static/CMS pages resolve dynamically: `/:lang/:staticPath` (legacy `/:lang/page/:key` kept for compatibility) — no hardcoded page list, resolved from `bootstrap.staticPages`.
 - Project Editor: `/edit/:section` or `/{lang}/edit/:section`.
-- Admin/backoffice: `/:lang/backoffice/**`, guarded by `adminAuthGuard` (`core/admin-auth/admin-auth.guard.ts`) — dashboard, products (fully wired), categories/static-pages/transactions/orders/media (routed to `BackofficeComingSoonPageComponent` placeholders pending features). See `docs/ADMIN.md`.
+- Admin/backoffice: `/:lang/backoffice/**`, guarded by `adminAuthGuard` (`core/admin-auth/admin-auth.guard.ts`) — dashboard, products, categories, orders, transactions, users, moderation, media, monitoring, analytics. See `docs/BACKEND.md` for the data-source contract for each.
 - Dev-only diagnostics: `/__diagnostics` (excluded from production).
 
 ## i18n system
@@ -51,4 +51,4 @@ src/app/
 
 ## Dynamic widget/section rendering from bootstrap JSON
 
-Full detail in `docs/ARCHITECTURE.md` and `docs/BACKEND_API.md#4-bootstrap`. Summary: `page config (bootstrap.pages) -> Section Engine (order/layout/visibility) -> Page Renderer -> Widget Host (resolves component via Widget Manifest + data via Data Source Resolver) -> widget component (props + resolved data only)`. Nothing in this pipeline calls an API directly except the Data Source Resolver, which delegates to `CategoryFacade`/`ProductFacade`.
+Full detail in `docs/ARCHITECTURE.md` and `docs/BACKEND.md#1-bootstrap`. Summary: `page config (bootstrap.pages) -> Section Engine (order/layout/visibility) -> Page Renderer -> Widget Host (resolves component via Widget Manifest + data via Data Source Resolver) -> widget component (props + resolved data only)`. Nothing in this pipeline calls an API directly except the Data Source Resolver, which delegates to `CategoryFacade`/`ProductFacade`.