feat(registratie): WP-35 — one Concept per case type (server-enforced)

Make "at most one unsubmitted Concept per type" a server invariant instead of a
client-only convenience. ApplicationStore.Create → CreateConcept guards atomically
under the write gate and POST /applications returns 409 when a duplicate would be
created. The FE draft-sync recovers from the 409 by adopting the existing Concept
(ensureId → findConcept) rather than erroring — one-per-type means the second
attempt lands on the existing draft. Typed client regenerated (documents the 409).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/project/backlog/README.md

@@ -79,7 +79,7 @@ for its existing violations, so every WP ends green.
 | [WP-32](WP-32-stamdata-undo.md)            | Undo/redo in the stamdata editor                                                  | 7 · refinements             | done   |
 | [WP-33](WP-33-dev-switchers.md)            | In-app dev switchers (scenario + role)                                            | 7 · refinements             | done   |
 | [WP-34](WP-34-adres-phone-brp-readonly.md) | Adres: phone field + BRP address read-only                                        | 7 · refinements             | done   |
-| [WP-35](WP-35-one-concept-per-type.md)     | One Concept per case type (server-enforced)                                       | 7 · refinements             | todo   |
+| [WP-35](WP-35-one-concept-per-type.md)     | One Concept per case type (server-enforced)                                       | 7 · refinements             | done   |
 | [WP-36](WP-36-admin-cases.md)              | Admin cases page + admin delete                                                   | 7 · refinements             | todo   |
 
 Sequencing dependencies (stated in the WPs too): 01 before 10–15 (axe covers story churn);

docs/project/backlog/WP-35-one-concept-per-type.md

@@ -0,0 +1,45 @@
+# WP-35 — One Concept per case type (server-enforced)
+
+Status: done
+Phase: 7 — refinements
+
+## Why
+
+The FE already keeps at most one Concept (draft aanvraag) per type — but only as a client-side
+convenience in `draft-sync.ts` (`resume()`/`findConcept()`/`resumeGate`). Per ADR-0001 the server
+is the authority for business rules; the FE guard is best-effort and a cross-tab / stale-list race
+can still POST a second Concept. This WP makes "at most one unsubmitted Concept per type" a
+**server-enforced invariant**, and makes the FE recover gracefully when the server refuses.
+
+## Decisions (made while building — no spec existed; flagged for review)
+
+- **Enforce at create, not submit.** The invariant is about the _existence_ of Concepts, so the
+  guard lives in `POST /applications`. Enforcing at submit would only block submitting a duplicate,
+  not its existence — that doesn't satisfy the title.
+- **Race-free in the store.** The check-and-insert happens atomically under the store's single
+  write gate (`ApplicationStore.CreateConcept`), not as a separate list-then-create in the handler.
+- **409 Conflict** (ProblemDetails), matching the applications block's other guards
+  (cancel-after-submit, submit-twice) — not 422. The generated client now handles 409 explicitly.
+- **FE recovery over error banner.** A create-409 means a Concept of this type already exists, so
+  `ensureId` adopts it (`findConcept`) instead of surfacing an error — the whole point of
+  one-per-type is that the second attempt lands you on the existing draft. Recovery fires only
+  when one actually exists; otherwise the original failure is surfaced.
+- **Scope: only the persisted-lifecycle types** (`registratie | herregistratie | intake`). The
+  stateless submits (`telefoonwijziging`, legacy `/registrations` etc.) never create a Concept.
+
+## Files
+
+- `backend/.../Data/ApplicationStore.cs` — `Create` → `CreateConcept` (nullable; atomic guard).
+- `backend/.../Program.cs` — `POST /applications` returns 409 when `CreateConcept` returns null.
+- `backend/tests/.../ApplicationTests.cs` — helper clears leftover Concepts (tests share one DB);
+  +3 tests (dup conflicts, different type allowed, new allowed after submit).
+- `src/app/registratie/application/draft-sync.ts` (+spec) — `ensureId` adopts the existing Concept
+  on a create-conflict.
+- Regenerated `api-client.ts` / `swagger.json` (create now documents its 409).
+
+## Acceptance criteria
+
+- [x] A second unsubmitted Concept of the same type is refused server-side (409).
+- [x] A different type, and a new Concept after the previous is submitted, are allowed.
+- [x] FE recovers from the 409 by resuming the existing Concept (no error banner).
+- [x] `npm run ci` green (333 FE tests, backend 125, api-client drift clean after commit).