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>

libs/shared/docs/a11y.mdx

@@ -65,5 +65,5 @@ the same way (`withInMemoryScrolling`), both wired once in `app.config.ts` — n
 fails it on a real axe violation. Both can be locally disabled — the lint rule via a
 normal ESLint disable comment, axe via `parameters: { a11y: { disable: true } }` — but
 only with a comment naming _why_ and a cross-reference to the WP expected to remove the
-skip (see `docs/project/backlog/WP-13-cibg-gap-register.md`'s marker convention, reused here).
+skip (see `docs/project/archive/backlog/WP-13-cibg-gap-register.md`'s marker convention, reused here).
 Grep `a11y: { disable: true }` in `*.stories.ts` for the current list.