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>

CLAUDE.md

@@ -12,7 +12,7 @@ signals. Auth is faked; **data and business rules are served by a minimal ASP.NE
 Core backend** (`backend/`, see its README) and consumed through an NSwag-generated
 typed client. The FE renders the backend's decisions. Reference data mimicking
 BRP/DUO (`Data/SeedData.cs`) is in-memory; applications, documents and the brief
-persist to a SQLite file via EF Core (WP-22) — `docs/project/backlog/WP-22-durable-persistence.md`.
+persist to a SQLite file via EF Core (WP-22) — `docs/project/archive/backlog/WP-22-durable-persistence.md`.
 
 **Monorepo (WP-67):** two Angular projects share one backend + one shared library —
 `apps/ssp` (Zorgverlener self-service, this doc's main subject) and `apps/behandelportal`
@@ -72,7 +72,7 @@ so this doesn't depend on a human remembering to run `/model` at the right momen
   `git status`/`grep`, verifying a file exists. No Edit/Write access.
 
 Delegate to the matching agent only when the _current_ session isn't already on that
-model — don't add indirection for its own sake. `docs/project/backlog/README.md`'s
+model — don't add indirection for its own sake. `docs/project/archive/backlog/README.md`'s
 session protocol is the worked example of this in practice.
 
 ## The decisions (non-negotiable working agreements)