feat(bff): committed OpenAPI contract + drift guard (refs #8)

Document typed responses (202 SubmitAccepted / 400 / 401 on self-service; 200
OpenbaarEntry[] on openbaar) so the generated spec carries real schemas for S-08's
client. A document transformer clears the auto-populated servers block so the spec
is host-independent and deterministic. Commit services/bff/openapi.json and add a
test asserting it matches the served /openapi/v1.json (fails on drift).

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

services/bff/Bff.Tests/OpenApiSpecTests.cs

@@ -0,0 +1,34 @@
+using System.Runtime.CompilerServices;
+using System.Text.Json;
+
+namespace Bff.Tests;
+
+/// <summary>
+/// Guards the committed OpenAPI contract (<c>services/bff/openapi.json</c>) against drift: it must
+/// equal the document the running BFF serves. S-08's Angular client is generated from this file, so a
+/// stale spec is a bug. To regenerate: run the BFF and save <c>/openapi/v1.json</c> over the file.
+/// </summary>
+public class OpenApiSpecTests
+{
+    [Fact]
+    public async Task Committed_openapi_spec_matches_the_served_document()
+    {
+        using var factory = new BffFactory();
+
+        var served = await factory.CreateClient().GetStringAsync("/openapi/v1.json");
+        var committed = await File.ReadAllTextAsync(SpecPath());
+
+        Assert.Equal(Canonical(committed), Canonical(served));
+    }
+
+    // Reformat both sides identically so the comparison is about content, not whitespace.
+    private static string Canonical(string json)
+    {
+        using var doc = JsonDocument.Parse(json);
+        return JsonSerializer.Serialize(doc.RootElement, new JsonSerializerOptions { WriteIndented = true });
+    }
+
+    // The committed spec sits at services/bff/openapi.json — one level up from this test file.
+    private static string SpecPath([CallerFilePath] string thisFile = "")
+        => Path.Combine(Path.GetDirectoryName(thisFile)!, "..", "openapi.json");
+}