docs: archive the finished backlogs (RD-30)

Two backlog trees are complete: `docs/project/backlog/` (75 files, every
WP done) and `docs/project/refactor-backlog-setup/` (the arc before it).
Move both under `docs/project/archive/` with `git mv`, so history stays
intact through `git log --follow`. `SHOWCASE-ROADMAP.md` moves with them,
because it points at the now-archived backlog README.

Add `docs/project/archive/README.md`. It states that these trees are
historical and names the two directories that are still live.

Repoint every inbound reference named in RD-30's Files table: CLAUDE.md,
the root README, both backend READMEs, `LetterHtml.cs`, `a11y.mdx`, the
`document-feature` and `new-ssp` skills, and the readable-codebase PLAN,
README, and RD-19 ticket. Fix two upward-relative links inside the moved
WP files (WP-68, WP-69) that gained a directory level and would otherwise
break. Repoint `.prettierignore`'s two agent-prompt exclusions to their
new path, so prettier keeps leaving those files' exact wording alone.

Mark RD-30 done and check off its acceptance criteria; flip its README
row to done.

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

docs/project/archive/backlog/WP-72-delete-legacy-submit-endpoints.md

@@ -0,0 +1,56 @@
+# WP-72 — Delete the dead legacy submit endpoints
+
+Status: done (6bc00a9)
+Phase: 12 — DDD hardening
+
+## Why
+
+`POST /api/v1/intakes` and `POST /api/v1/herregistraties` were dead from the UI — the wizard
+submits through `POST /applications/{id}/submit`, and nothing in `apps/` or `libs/` called the
+generated `intakes()`/`herregistraties()` client methods. They were also strictly **less
+capable** than the endpoint that replaced them: they minted a bare reference and wrote no
+`Aanvraag`, made no ZGW/OpenZaak call, and performed no document-ownership check.
+
+WP-69 hardened `/intakes` with a 400 last session. Deleting the surface is the stronger fix;
+WP-69's `/applications/{id}/submit` enforcement — the path the wizard actually uses — is
+untouched.
+
+## Decisions (pre-made)
+
+1. Delete both routes together. Their two `EndpointTests` are `[Theory]`s parameterised across
+   _both_ routes, so deleting one would leave an `InlineData` row 404-ing.
+2. **Keep** the shared `Submit(...)` helper, `ReferentieResponse`, `SubmissionRules.NewReference`
+   and the whole `IdempotencyStore` path — `/registrations` and `/change-requests` still use
+   them, and `IdempotencyTests` covers the latter.
+3. This WP owns the wire artifacts; no other track runs `gen:api`.
+
+## Acceptance criteria
+
+- [x] Both routes return **404** against a live backend (verified by curl, not by inference).
+- [x] Zero references remain in `libs/shared/src/infrastructure/api-client.ts`.
+- [x] `gen:api` diff is **pure deletion** — 124 lines out of `swagger.json`, 109 out of the API
+      client, zero additions.
+- [x] Backend tests 245 → 240, exactly the 5 deleted cases (2 `[Theory]`s × 2 rows + 1 `[Fact]`).
+- [x] `Submit(...)` and the idempotency path survive with their live callers intact.
+
+## Verification
+
+```bash
+cd backend && dotnet test BigRegister.slnx --filter "Category!=Integration"
+npm run ci
+curl -X POST http://localhost:5000/api/v1/intakes -d '{"uren":500}'   # 404
+```
+
+## Notes
+
+Committed together with WP-73 (`6bc00a9`): both edit `Program.cs`, and splitting them would
+have produced a commit that does not build. The two were run in separate execution waves to
+avoid a concurrent `dotnet build` collision — but since neither committed independently, the
+file-level entanglement remained at integration time. Worth remembering when planning future
+parallel backend tracks: **separate waves do not produce separate commits.**
+
+## Follow-ups
+
+- `docs/reference/fp-tea-atomic-design.md:587` / `ARCHITECTURE.md:464` still teach a
+  `visibleSteps`-with-a-`'scholing'`-step intake the fixed-3-step wizard no longer matches
+  (inherited from WP-69).