feat(backend): enforce the scholing threshold server-side (WP-69)

ADR-0001's own canonical "config value" example was unenforced: GET
/intake/policy echoed ScholingThreshold, but no request DTO carried a
scholing answer, so the server had nothing to re-validate. A crafted
POST could skip a requirement the wizard presents as mandatory.

IntakePolicy.RejectIncompleteScholing is the authority — three-valued
completeness (below threshold an answer is required; "nee" is legal and
still submits; punten only belong to a followed scholing), living in the
class that owns the constant so scripts/check-seam.sh keeps guarding the
FE/BE literal pair. Both submit paths call it; a violation 400s with
ProblemDetails and leaves the aanvraag a Concept. Gated on
Type == "intake" (the endpoint's switch lumps herregistratie with
intake, which has no scholing question), and guarded by `reject is null`
so a zero-uren submission is still decided on its merits.

Also fixes a live FE bug in the same rule: validateStep required punten
whenever scholingGevolgd was 'ja' regardless of lageUren, while the
template renders those fields only when lageUren — so answering 'ja'
then raising uren either blocked the user on an invisible field or
emitted aanvullendeScholing: undefined alongside punten. punten now
derives from aanvullendeScholing, so that combination is unrepresentable
in ValidIntake.

Note: EndpointTests' Worked_hours_submission_succeeds was itself
asserting the vulnerable payload ({ uren: 40 }, no answer) and needed a
complete answer added; the zero-hours rows are the ordering regression
net and are unmodified.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

libs/shared/src/infrastructure/api-client.ts

@@ -483,6 +483,12 @@ export class ApiClient {
             result200 = _responseText === "" ? null : JSON.parse(_responseText, this.jsonParseReviver) as ReferentieResponse;
             return result200;
             });
+        } else if (status === 400) {
+            return response.text().then((_responseText) => {
+            let result400: any = null;
+            result400 = _responseText === "" ? null : JSON.parse(_responseText, this.jsonParseReviver) as ProblemDetails;
+            return throwException("Bad Request", status, _responseText, _headers, result400);
+            });
         } else if (status === 422) {
             return response.text().then((_responseText) => {
             let result422: any = null;
@@ -2210,6 +2216,8 @@ export interface IntakePolicyDto {
 
 export interface IntakeRequest {
     uren?: number;
+    aanvullendeScholing?: boolean | undefined;
+    scholingPunten?: number | undefined;
 }
 
 export interface LetterBlockDto {
@@ -2419,6 +2427,8 @@ export interface SubmitApplicationRequest {
     diplomaHerkomst?: string | undefined;
     uren?: number | undefined;
     documents?: DocumentRefDto[] | undefined;
+    aanvullendeScholing?: boolean | undefined;
+    scholingPunten?: number | undefined;
 }
 
 export interface SubmitApplicationResponse {