docs: consolidate scattered docs into canonical set

Replace ~35 organically-grown docs (docs/platform/*, docs/backend-platform/*,
one-off sprint reports, Search.md, Diagnostics.md, Content-Management.md,
Backend-Handoff-Sprint16.md, docs/superpowers/*, docs/Project-Editor.md,
untracked docs/total.md) with the six canonical docs declared in
.claude/CLAUDE.md: PROJECT.md, ARCHITECTURE.md, BACKEND.md, FRONTEND.md,
BOOTSTRAP.md, EDITOR.md, plus a new PROJECT-STRUCTURE.md.

- BACKEND.md is a punch list per domain (auth, bootstrap draft/publish,
  static pages, categories, products, orders, dashboard metrics, activity,
  translations, search, product engagement) plus a Known reliability issues
  section on the prod 502/504 root cause.
- ARCHITECTURE.md links to (does not duplicate) the enforced
  docs/architecture/foundation/** ADRs and standards docs.
- docs/ADMIN.md and docs/architecture/foundation/** and docs/context/** are
  left untouched per instructions.
- Updated the one dangling docs/Project-Editor.md reference in
  admin-auth.service.ts to point at docs/BACKEND.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/core/admin-auth/admin-auth.service.ts

@@ -12,7 +12,7 @@ import { TelegramSessionApiService } from '../../services/telegram-session-api.s
  * never authenticates the customer session or vice versa: distinct cookie
  * name, distinct signals, distinct guard/interceptor.
  *
- * Backend gap this creates (see docs/Project-Editor.md): since the session
+ * Backend gap this creates (see docs/BACKEND.md, item 1): since the session
  * API itself has no concept of "admin", the frontend cannot tell an admin
  * Telegram session from a regular one. Actual admin authorization must be
  * enforced server-side when admin API calls are made with the resulting