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/swagger.json

@@ -314,6 +314,16 @@
               }
             }
           },
+          "400": {
+            "description": "Bad Request",
+            "content": {
+              "application/problem+json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          },
           "422": {
             "description": "Unprocessable Content",
             "content": {
@@ -2172,6 +2182,15 @@
           "uren": {
             "type": "integer",
             "format": "int32"
+          },
+          "aanvullendeScholing": {
+            "type": "boolean",
+            "nullable": true
+          },
+          "scholingPunten": {
+            "type": "integer",
+            "format": "int32",
+            "nullable": true
           }
         },
         "additionalProperties": false
@@ -2837,6 +2856,15 @@
               "$ref": "#/components/schemas/DocumentRefDto"
             },
             "nullable": true
+          },
+          "aanvullendeScholing": {
+            "type": "boolean",
+            "nullable": true
+          },
+          "scholingPunten": {
+            "type": "integer",
+            "format": "int32",
+            "nullable": true
           }
         },
         "additionalProperties": false