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>

docs/reference/architecture/0001-bff-lite-decision-dtos.md

@@ -101,6 +101,10 @@ would compute. Two slices were implemented to demonstrate **both** policy shapes
   machine state and is set via a `SetPolicy` message. A `SCHOLING_THRESHOLD_DEFAULT`
   remains only as the offline fallback.
 - `intake-wizard.component.ts` fetches the policy and dispatches `SetPolicy`.
+- WP-69: the backend re-validates the threshold as the authority on submit —
+  `IntakePolicy.RejectIncompleteScholing` runs before `POST /applications/{id}/submit`
+  (intake-typed) and the legacy `POST /intakes` write anything, 400ing an incomplete
+  scholing answer instead of silently accepting a crafted POST that skips it.
 
 ## Migration sequence (for the real app)