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-60-write-divergence-resilience.md

@@ -0,0 +1,94 @@
+# WP-60 — Write-divergence resilience (local + ZGW writes)
+
+Status: done
+Phase: 10 — OpenZaak production hardening
+
+## Why
+
+A citizen action today does a local `Aanvraag`/`Document` write and a paired ZGW write
+(create zaak/status/document); these aren't transactional. If the ZGW call fails after the
+local write succeeds (or vice versa), the two diverge silently —
+`openzaak-integration.md` flags this explicitly as "acceptable for a demo backend; a
+production arc needs retry/reconciliation or an outbox." This is the one genuine
+correctness gap standing between the current integration and something safe to call
+production.
+
+## Read first
+
+- [openzaak-integration.md](../reference/openzaak-integration.md) (the section discussing
+  this gap)
+- `backend/src/BigRegister.Api/Data/ApplicationStore.cs`,
+  `Zgw/OpenZaakZaakSource.cs` (the two write sides)
+- [ADR-0005 — OpenZaak behind the BFF](../reference/architecture/0005-openzaak-behind-bff.md)
+
+## Decisions
+
+Picked **(b), narrowed further: bounded synchronous retry + flag, no reconcile job.** The
+`planner` agent's kickoff review found the write side smaller than either option assumed:
+
+- The only ZGW writes are `OpenZaakZaakSource.CreateZaak` (zaak/status/rol, one POST sequence
+  per submit) and `OpenZaakDocumentSource.Upload`/`LinkToZaak` (DRC + zaakinformatieobject).
+  There is no standalone status-transition write path yet (that's WP-66) — Step 2 below is
+  corrected accordingly.
+- Every path already does the local write first and never rolls it back on a ZGW failure — "the
+  ZGW half fails, local succeeded" is the only real scenario; the reverse can't happen.
+- An outbox was rejected: three request-triggered write paths don't justify a persisted queue,
+  and a ZGW call's `CallerIdentity` (needed for the JWT's audit claims, WP-53) would mean PII
+  sitting in a new table — the "generic outbox framework" this WP's own Risks section warns
+  against.
+- A reconcile job was judged unnecessary for the acceptance criteria: flagging (not silent
+  divergence) is sufficient, and repair is always possible on demand because a zaak's
+  `identificatie` equals the aanvraag's `Referentie` — no reconcile job ships in this WP.
+
+Shipped: bounded retry (3 attempts, doubling backoff from 200ms) in `ZgwHttpClient` for
+transport-shaped failures only (429/502/503/504/408 + connection errors/timeouts — deliberately
+**not** 500, which can follow a partial commit on the non-idempotent `/statussen`/`/rollen`
+POSTs); `Aanvraag.ZgwError` + a `zgw:divergence` audit row when a ZGW write still fails after
+retry (`Program.cs`'s submit endpoint, two separate try/catches so a create-zaak failure doesn't
+also skip the still-local document link); `OpenZaakDocumentSource.Upload` catches and logs
+without a separate flag column (`DrcUrl == null` already means "not registered in ZGW yet").
+Full reasoning + rejected sub-options: [openzaak-integration.md](../reference/openzaak-integration.md)'s
+"Write resilience" section.
+
+## Files
+
+`Zgw/ZgwHttpClient.cs` (retry), `Data/ApplicationStore.cs` (`ZgwError` column + migration),
+`Program.cs` (submit endpoint rewire + `RecordZgwDivergence` + HttpClient timeouts),
+`Zgw/OpenZaakDocumentSource.cs` (non-throwing upload). No new file for a mechanism — no
+outbox/background worker shipped (see Decisions).
+
+## Steps
+
+1. Design review with the `planner` agent — pick outbox vs. retry+reconcile. Done: retry+flag
+   (see Decisions).
+2. Implement the chosen mechanism for the create-zaak and document (upload + link) write
+   paths — not "status-transition" as originally scoped here; that path doesn't exist yet
+   (arrives with WP-66).
+3. Add a test that simulates a ZGW failure mid-write and asserts the system recovers
+   (retries successfully, or is left in a detectably-inconsistent-but-flagged state)
+   rather than silently diverging.
+
+## Acceptance criteria
+
+- [x] A simulated ZGW failure after a successful local write no longer leaves permanent
+      silent divergence — either it retries to consistency or the divergence is
+      detectable/flagged.
+- [x] No new synchronous latency added to the happy path beyond what the chosen mechanism
+      requires.
+
+## Verification
+
+A new integration test that fails a stubbed ZGW call mid-write and asserts
+recovery/flagging behavior; `cd backend && dotnet test`.
+
+## Out of scope
+
+A general-purpose outbox framework reusable beyond this one write pair (YAGNI unless a
+second write pair appears — note WP-66 is exactly that second pair, so revisit scope if
+WP-66 lands first); UI surfacing of reconciliation state (backend-only fix for now).
+
+## Risks
+
+Over-building this (a generic outbox/saga framework) for a POC's actual write volume —
+ladder check at kickoff: does a bounded retry + periodic reconcile job cover it before
+reaching for an outbox table?