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.IntegrationTests/OpenZaakGatewayIntegrationTests.cs

@@ -120,7 +120,7 @@ public sealed class OpenZaakGatewayIntegrationTests(OpenZaakFixture stack)
         Assert.Equal(content.Length, doc.GetProperty("bestandsomvang").GetInt32());
         // indicatieGebruiksrecht is recorded as "no restrictions"; left null, OpenZaak would refuse to
         // close the zaak this document is related to (the S-10b regression that broke the e2e flow).
-        Assert.False(doc.GetProperty("indicatiegebruiksrecht").GetBoolean());
+        Assert.False(doc.GetProperty("indicatieGebruiksrecht").GetBoolean());
 
         // ...and it is related to the zaak (a zaakinformatieobject links the two).
         var relations = await stack.GetJsonAsync(new Uri(stack.BaseUrl,