feat(fp): WP-21 — resilience seams (correlation-id, idempotency, retry)

Correlation id becomes real ASP.NET Core middleware instead of a per-endpoint
read: every request gets one (client-supplied or generated), it's echoed as
an X-Correlation-Id response header, and pushed into the logging scope so
every log line for that request carries it — not just the Submit helper's,
verified against LogBrief which never threads it explicitly.

Idempotency-Key moves from per-HTTP-attempt (defeating its own purpose) to
per-logical-submit: runSubmit mints one key and threads it through a small
bridge (withIdempotencyKey/currentIdempotencyKey) since the NSwag-generated
client has no per-call header hook. Backend gains an IdempotencyStore that
short-circuits a replayed key to the first call's result instead of minting
a second reference — scoped to the Submit-helper endpoints per the WP's own
decision.

GET requests now retry transient failures (rxjs retry({count:2, delay:500}));
writes never auto-retry. Proven with a fake-HttpClient spec
(api-client.provider.spec.ts) rather than a manual network-tab check — the
WP's suggested `?scenario=error` check turned out not to exercise a real
network call at all (the interceptor throws before calling next()), so the
automated test is the actual proof.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/BigRegister.Tests/IdempotencyTests.cs

@@ -0,0 +1,69 @@
+using System.Net.Http.Json;
+using BigRegister.Api.Contracts;
+using Microsoft.AspNetCore.Mvc.Testing;
+
+namespace BigRegister.Tests;
+
+public class IdempotencyTests(WebApplicationFactory<Program> factory) : IClassFixture<WebApplicationFactory<Program>>
+{
+  private readonly HttpClient _client = factory.CreateClient();
+
+  private static HttpRequestMessage ChangeRequestWithKey(string key)
+  {
+    var req = new HttpRequestMessage(HttpMethod.Post, "/api/v1/change-requests")
+    {
+      Content = JsonContent.Create(new { straat = "Lange Voorhout 9", postcode = "2514 EA", woonplaats = "Den Haag" }),
+    };
+    req.Headers.Add("Idempotency-Key", key);
+    return req;
+  }
+
+  [Fact]
+  public async Task Replaying_the_same_idempotency_key_returns_the_same_reference_not_a_new_one()
+  {
+    var key = Guid.NewGuid().ToString();
+
+    var first = await _client.SendAsync(ChangeRequestWithKey(key));
+    first.EnsureSuccessStatusCode();
+    var firstBody = await first.Content.ReadFromJsonAsync<ReferentieResponse>();
+
+    var replay = await _client.SendAsync(ChangeRequestWithKey(key));
+    replay.EnsureSuccessStatusCode();
+    var replayBody = await replay.Content.ReadFromJsonAsync<ReferentieResponse>();
+
+    Assert.Equal(firstBody!.Referentie, replayBody!.Referentie);
+  }
+
+  [Fact]
+  public async Task Different_idempotency_keys_are_independent_submissions()
+  {
+    var first = await _client.SendAsync(ChangeRequestWithKey(Guid.NewGuid().ToString()));
+    var second = await _client.SendAsync(ChangeRequestWithKey(Guid.NewGuid().ToString()));
+    var firstBody = await first.Content.ReadFromJsonAsync<ReferentieResponse>();
+    var secondBody = await second.Content.ReadFromJsonAsync<ReferentieResponse>();
+
+    Assert.NotEqual(firstBody!.Referentie, secondBody!.Referentie);
+  }
+
+  [Fact]
+  public async Task A_rejected_submission_replays_the_same_rejection_not_a_retry()
+  {
+    var key = Guid.NewGuid().ToString();
+    var badRequest = new HttpRequestMessage(HttpMethod.Post, "/api/v1/change-requests")
+    {
+      Content = JsonContent.Create(new { straat = "Straat 1", postcode = "nope", woonplaats = "Den Haag" }),
+    };
+    badRequest.Headers.Add("Idempotency-Key", key);
+
+    var first = await _client.SendAsync(badRequest);
+    Assert.Equal(System.Net.HttpStatusCode.UnprocessableEntity, first.StatusCode);
+
+    var replayRequest = new HttpRequestMessage(HttpMethod.Post, "/api/v1/change-requests")
+    {
+      Content = JsonContent.Create(new { straat = "Straat 1", postcode = "nope", woonplaats = "Den Haag" }),
+    };
+    replayRequest.Headers.Add("Idempotency-Key", key);
+    var replay = await _client.SendAsync(replayRequest);
+    Assert.Equal(System.Net.HttpStatusCode.UnprocessableEntity, replay.StatusCode);
+  }
+}