fix(acl): use camelCase indicatieGebruiksrecht so OpenZaak honours it (refs #103)

The ZGW Documenten API property is indicatieGebruiksrecht (camelCase); the
all-lowercase key was silently dropped on write (DRF camel-case mapping),
leaving the field null so the zaak still could not close. Correct the JSON
name on the DTO and both assertions to match the ZGW spec.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/acl/Acl.Tests/OpenZaakGatewayTests.cs

@@ -491,7 +491,7 @@ public class OpenZaakGatewayTests
         Assert.Contains("\"status\":\"definitief\"", create.Body);
         // indicatieGebruiksrecht must be set explicitly (false = no usage restrictions); left null,
         // OpenZaak refuses to close the zaak this document is related to ("indicatiegebruiksrecht-unset").
-        Assert.Contains("\"indicatiegebruiksrecht\":false", create.Body);
+        Assert.Contains("\"indicatieGebruiksrecht\":false", create.Body);
         // The file content is base64-encoded into `inhoud`, with its byte length in `bestandsomvang`.
         Assert.Contains($"\"inhoud\":\"{Convert.ToBase64String([10, 20, 30])}\"", create.Body);
         Assert.Contains("\"bestandsomvang\":3", create.Body);