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/KNOWN-ISSUES.md

@@ -1,6 +1,6 @@
 # Known Issues
 
-Real, reproducible, currently-open frontend bugs only. Everything that needed a product/business decision moved to `docs/PRODUCT_BACKLOG.md`; everything nice-to-have moved to `docs/FUTURE_FEATURES.md`; everything backend-shaped moved to `docs/BACKEND_INTEGRATION.md`. Re-verified against source 2026-07-26.
+Real, reproducible, currently-open frontend bugs only. Everything that needed a product/business decision moved to `docs/PRODUCT_BACKLOG.md`; everything nice-to-have moved to `docs/FUTURE_FEATURES.md`; everything backend-shaped moved to `docs/BACKEND.md`. Re-verified against source 2026-07-26.
 
 ## Open
 
@@ -17,11 +17,11 @@ Real, reproducible, currently-open frontend bugs only. Everything that needed a
    `'invalid-signature'`. Both screens exist and are wired, but are permanently
    unreachable from any real backend response today.
    - **Fix requires both sides**: a backend that returns a distinguishable
-     `error.code` in the response body (see `docs/ERROR_CONTRACT.md`), and a small
+     `error.code` in the response body (see `docs/BACKEND.md` §6 Error Model), and a small
      frontend change to `toAuthErrorShape()` to prefer that body code over the
      blanket status-based fallback.
    - Found: 2026-07-26, Backend Finalization Sprint documentation pass (traced while
-     writing `docs/AUTHENTICATION.md`/`docs/ERROR_CONTRACT.md`).
+     writing `docs/BACKEND.md` §4 Authentication / §6 Error Model).
 
 ## Fixed (this cycle)