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/README.md

@@ -0,0 +1,11 @@
+# Archive — finished backlogs
+
+The trees in this directory are historical. Every ticket in `backlog/` and
+`refactor-backlog-setup/` carries `Status: done`. `SHOWCASE-ROADMAP.md` was superseded before
+either arc started.
+
+Git holds the rest — `git log --follow` on any file here shows its full history, including the
+work that produced it.
+
+The live plan is `docs/project/readable-codebase/`. Product reference material is
+`docs/project/prd/`.

docs/project/archive/SHOWCASE-ROADMAP.md

@@ -1,6 +1,6 @@
 # Showcase roadmap — superseded
 
-**This roadmap is superseded by [`docs/project/backlog/`](backlog/README.md)** (2026-07-02).
+**This roadmap is superseded by [`docs/project/archive/backlog/`](backlog/README.md)** (2026-07-02).
 
 The backlog absorbs and corrects this document: its Storybook-as-curriculum track became
 WP-14/15 (+ per-invariant MDX pages in WP-05/07/08/13/17), its enforcement track became

docs/project/archive/backlog/WP-68-ddd-aggregate-hardening.md

@@ -59,7 +59,7 @@ The review's remaining findings are listed under "Follow-ups" and are **not** th
 ## Read first
 
 - `CLAUDE.md` §"The decisions" #3 (make illegal states unrepresentable) and #4 (BFF-lite)
-- [ADR-0001 — BFF-lite + decision DTOs](../../reference/architecture/0001-bff-lite-decision-dtos.md)
+- [ADR-0001 — BFF-lite + decision DTOs](../../../reference/architecture/0001-bff-lite-decision-dtos.md)
 - `backend/src/BigRegister.Api/Data/ApplicationStore.cs` (the `Aanvraag` entity, the store's
   lock discipline, `AanvraagStatusTag`, `RecordBesluit`)
 - `backend/src/BigRegister.Api/Contracts/Mappers.cs` (`ToStatusDto` — the logic to move)

docs/project/archive/backlog/WP-69-intake-scholing-threshold-enforcement.md

@@ -39,7 +39,7 @@ re-validates as authority) is unenforced for the one rule it was written to illu
 
 - `backend/src/BigRegister.Api/Domain/Intake/IntakePolicy.cs` (the corrected doc-comment,
   WP-68)
-- [ADR-0001 — BFF-lite + decision DTOs](../../reference/architecture/0001-bff-lite-decision-dtos.md)
+- [ADR-0001 — BFF-lite + decision DTOs](../../../reference/architecture/0001-bff-lite-decision-dtos.md)
   §"config value"
 - `apps/ssp/src/app/herregistratie/domain/intake.machine.ts` (`lageUren`, `scholingGevolgd`,
   `punten` — the wizard's existing FE-side rule and its answers)