fix(workflow): read task query variables from 'variables', not 'processVariables' (refs #13)

Flowable's POST query/tasks returns the included process variables under the key
'variables' (the request opts in via includeProcessVariables). The client DTO, its
unit test, and the live-check parser all looked for 'processVariables', so the
werkbak never matched a task's registrationId and verify-domain timed out. Verified
by driving a real Flowable instance end-to-end locally: start -> complete external
job -> the Beoordelen task carries registrationId under 'variables'.

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

services/domain/Big.Infrastructure/FlowableWorkflowClient.cs

@@ -137,11 +137,13 @@ public sealed class FlowableWorkflowClient(HttpClient http, FlowableOptions opti
 
     private sealed record UserTaskDto(
         [property: JsonPropertyName("id")] string Id,
-        [property: JsonPropertyName("processVariables")] IReadOnlyList<Variable>? ProcessVariables)
+        // Flowable's task-query returns the (included) process variables under "variables", not
+        // "processVariables"; the request opts in via includeProcessVariables.
+        [property: JsonPropertyName("variables")] IReadOnlyList<Variable>? Variables)
     {
         /// <summary>The registration id this task carries as a process variable.</summary>
         public string RegistrationId() =>
-            ProcessVariables?.SingleOrDefault(v => v.Name == "registrationId")?.Value
+            Variables?.SingleOrDefault(v => v.Name == "registrationId")?.Value
             ?? throw new InvalidOperationException($"Beoordelen task {Id} carries no registrationId variable.");
     }