fix(infra): address Objecten by a dotted host so NRC accepts its notifications (refs #152)

The worker published and NRC answered 400 on every message:

  {"hoofdObject":["Voer een geldige URL in."],"resourceUrl":["Voer een geldige URL in."]}

NRC types both as DRF `URLField`, and Django's URLValidator refuses a single-label host.
Objecten fills them from the object `url` DRF built with `request.build_absolute_uri` —
the Host the *caller* used — so `SITE_DOMAIN` never entered into it. Dropped that env pair;
it was a wrong guess at the mechanism.

The fix is on the caller side: keep the `objecten.local` network alias and point every
writer whose writes must be notified at it — the ACL, the gateway integration tests, and
this slice's verify driver. Readers keep the plain service name.

ADR-0029 updated with the real mechanism and the ceiling it leaves: a new writer using
`objecten:8000` gets a 201 and silently no notification.

services/acl/Acl.IntegrationTests/ObjectenGatewayIntegrationTests.cs

@@ -20,7 +20,7 @@ public sealed class ObjectenGatewayIntegrationTests
         new HttpClient(),
         new ObjectenOptions
         {
-            BaseUrl = new(Env("OBJECTEN_BASE", "http://objecten:8000")),
+            BaseUrl = new(Env("OBJECTEN_BASE", "http://objecten.local:8000")),
             Token = Env("OBJECTEN_TOKEN", "1234567890abcdef1234567890abcdef12345678"),
             ObjecttypenBaseUrl = new(Env("OBJECTTYPEN_BASE", "http://objecttypen:8000")),
             ObjecttypenToken = Env("OBJECTTYPEN_TOKEN", "0123456789abcdef0123456789abcdef01234567"),
@@ -65,7 +65,7 @@ public sealed class ObjectenGatewayIntegrationTests
     {
         using var http = new HttpClient();
         var objecttype = await ResolveObjecttypeUrlAsync(http);
-        var query = new Uri(new Uri(Env("OBJECTEN_BASE", "http://objecten:8000")),
+        var query = new Uri(new Uri(Env("OBJECTEN_BASE", "http://objecten.local:8000")),
             "/api/v2/objects?type=" + Uri.EscapeDataString(objecttype) +
             "&data_attrs=id__exact__" + Uri.EscapeDataString(id));