feat(behandelportal): WP-65b beoordeling besluit (decision write)

Adds POST /beoordeling/{id}/besluit: a Besluit enum (Goedkeuren/Afwijzen/
MeerInfoOpvragen) backed by new Aanvraag.BesluitStatus/BesluitToelichting
columns, gated by the same BeoordelingRules.CanDecide the read side's
canBesluiten flag already uses (409 on an illegal transition, 400 on a
missing required toelichting). Mappers.ToStatusDto gains the "a recorded
decision wins" branch. FE: besluit.machine.ts + besluit-form organism
(same form idiom as change-request-form), wired into the beoordeling page
behind the server's canBesluiten flag.

Completes WP-65 (65a + 65b) — verified end-to-end against a running
backend (werkvoorraad -> beoordeling -> besluit -> status reflected back).

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

backend/src/BigRegister.Api/Contracts/Dtos.cs

@@ -140,6 +140,15 @@ public sealed record BeoordelingViewDto(
     IReadOnlyList<BeoordelingDocumentDto> Documenten,
     BeoordelingDecisionsDto Decisions);
 
+/// Recording a decision (WP-65b). `Besluit` is the enum member name as a string — same
+/// wire convention as `AanvraagStatusDto.Tag` (this backend never ships a raw C# enum,
+/// it round-trips names via Enum.Parse/.ToString() at the Contracts boundary, no
+/// JsonStringEnumConverter configured). The endpoint 400s an unknown name. Toelichting
+/// is required for Afwijzen/MeerInfoOpvragen, validated server-side.
+public sealed record RecordBesluitRequest(string Besluit, string? Toelichting = null);
+
+public sealed record RecordBesluitResponse(AanvraagStatusDto Status);
+
 // --- Brief (letter composition) contracts ---
 // Rich text is a serialisable node tree; the node union is flattened with a `Type`
 // discriminator + nullable fields, the same wire convention as AanvraagStatusDto.