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/EndpointTests.cs

@@ -125,6 +125,22 @@ public class EndpointTests(WebApplicationFactory<Program> factory) : IClassFixtu
     res.EnsureSuccessStatusCode();
   }
 
+  [Fact]
+  public async Task Correlation_id_supplied_by_the_caller_is_echoed_back()
+  {
+    var req = new HttpRequestMessage(HttpMethod.Get, "/api/v1/notes");
+    req.Headers.Add("X-Correlation-Id", "test-cid-123");
+    var res = await _client.SendAsync(req);
+    Assert.Equal("test-cid-123", res.Headers.GetValues("X-Correlation-Id").Single());
+  }
+
+  [Fact]
+  public async Task Correlation_id_is_generated_when_the_caller_omits_it()
+  {
+    var res = await _client.GetAsync("/api/v1/notes");
+    Assert.NotEmpty(res.Headers.GetValues("X-Correlation-Id").Single());
+  }
+
   // --- Document upload ---
 
   private static MultipartFormDataContent UploadForm(string localId, string categoryId, string wizardId, string fileName, string contentType)