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>

backend/tests/BigRegister.Tests/EndpointTests.cs

@@ -103,7 +103,10 @@ public class EndpointTests(TestWebApplicationFactory factory) : IClassFixture<Te
   [InlineData("/api/v1/herregistraties")]
   public async Task Worked_hours_submission_succeeds(string route)
   {
-    var res = await _client.PostAsJsonAsync(route, new { uren = 40 });
+    // WP-69: 40 is below IntakePolicy.ScholingThreshold, so /intakes now requires the
+    // scholing question answered — `aanvullendeScholing` is unknown to (and ignored by)
+    // HerregistratieRequest, so this one extra field keeps serving both rows unchanged.
+    var res = await _client.PostAsJsonAsync(route, new { uren = 40, aanvullendeScholing = false });
     res.EnsureSuccessStatusCode();
   }