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);