fix(backend): 400 instead of 500 on an unparseable peildatum (RB-16)

BIO-019: GET /stamdata/{table}?peildatum= called DateOnly.Parse
directly, which throws FormatException on anything unparseable — an
unhandled 500 (leaking exception detail in Development) instead of
the 400-with-problem-details every other bad-input check in this file
returns. §3c named backend/Stamdata's 71.7% branch coverage (BL-005)
as the weak spot this bug lived in.

Switched to DateOnly.TryParse; an unparseable value now returns
Results.Problem(detail: ..., statusCode: 400), matching the shape the
upload/change-request endpoints already use. Endpoint doc gained
.ProducesProblem(400), so the OpenAPI doc + generated client were
regenerated and committed in this same diff (RB-09's note records a
prior incident where a response-shape change shipped without this and
the drift went unnoticed).

No FE change needed: libs/beheer's stamdata adapter already funnels
every call through runSubmit, which folds any thrown ApiException
(now including this 400) into a generic Result error — ADR-0001's
"the FE renders the decision" already covers "the server rejected
this input".

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

backend/swagger.json

@@ -194,6 +194,16 @@
               }
             }
           },
+          "400": {
+            "description": "Bad Request",
+            "content": {
+              "application/problem+json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          },
           "403": {
             "description": "Forbidden",
             "content": {