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/PROJECT-STRUCTURE.md

@@ -28,7 +28,7 @@ Standards referenced below are enforced, not suggestions: `docs/architecture/fou
 
 2. **Gateway interface** — `services/admin-dashboard-metrics.gateway.interface.ts`. An abstract contract (`AdminDashboardMetricsGateway`) for "however we get category/product counts" — deliberately decoupled from *how* (local computation vs. real API) so the facade never knows which implementation is active.
 
-3. **Gateway implementation** — `services/admin-dashboard-metrics.local.gateway.ts`. `AdminDashboardMetricsLocalGateway implements AdminDashboardMetricsGateway`, composing `BackofficeDataService.loadCategories()/loadProducts()` (already used elsewhere) into counts. A future `AdminDashboardMetricsApiGateway` would implement the same interface against a real endpoint (`docs/BACKEND_API.md#615-backoffice--dashboard-metrics--recent-activity-planned`) — nothing above this layer changes when that happens.
+3. **Gateway implementation** — `services/admin-dashboard-metrics.local.gateway.ts`. `AdminDashboardMetricsLocalGateway implements AdminDashboardMetricsGateway`, composing `BackofficeDataService.loadCategories()/loadProducts()` (already used elsewhere) into counts. A future `AdminDashboardMetricsApiGateway` would implement the same interface against a real endpoint (see `docs/BACKEND.md` §3 CRUD Contracts / §8 migration guide) — nothing above this layer changes when that happens.
 
 4. **DI token** — `services/admin-dashboard-metrics-gateway.token.ts`. `const ADMIN_DASHBOARD_METRICS_GATEWAY = new InjectionToken<AdminDashboardMetricsGateway>(...)`, bound to the local gateway by default in `app.config.ts`. This is the swap point: rebinding this token to a real API gateway is the *only* change needed to go from mock to real data.
 
@@ -42,7 +42,7 @@ Standards referenced below are enforced, not suggestions: `docs/architecture/fou
 
 9. **Route wiring** — `app.routes.ts`. `/:lang/backoffice/dashboard -> AdminDashboardPageComponent`, guarded by `adminAuthGuard`; `/:lang/backoffice` (empty path) redirects to `dashboard`.
 
-Full narrative and known gaps: `docs/ADMIN.md`.
+Full narrative and known gaps: `docs/archive/ADMIN.md` (historical build log) and `docs/BACKEND.md` (current contract).
 
 ## Steps to add a new feature (derived from the example above)
 
@@ -56,5 +56,5 @@ Full narrative and known gaps: `docs/ADMIN.md`.
 8. Build the container/page component that injects the facade and wires routing.
 9. Add routes in `app.routes.ts`, with `adminAuthGuard` (or the relevant guard) if it's an admin surface.
 10. Add every new user-facing string to `i18n/translations.ts` (interface) then `en.ts`/`ru.ts`/`hy.ts` — never hardcode copy in a template.
-11. Document backend gaps (if any) in `docs/BACKEND_API.md` (§6, endpoints by domain) using the same CURRENT/PLANNED/FUTURE tagging as the existing entries.
+11. Document backend gaps (if any) in `docs/BACKEND.md` §3 (CRUD Contracts, endpoints by domain), marking proposed/unimplemented endpoints as such.
 12. Run `npm run arch:check` (import boundaries + circular dependencies) and `npx tsc -p tsconfig.app.json --noEmit` before committing.