fix(backend): key IdempotencyStore on caller + idem key (RB-18)

IdempotencyStore keyed a replayed submission on the raw Idempotency-Key
header alone. Two different callers who send the same header value
shared one cache slot: the second caller received the first caller's
cached reference instead of running its own submission.

Program.cs now composes the key as "{SubjectId}:{idemKey}" in the
Submit helper, so the cache is scoped per caller. Add a test that
proves a caller cannot replay another caller's idempotency key and
receive their cached result.

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

backend/tests/BigRegister.Tests/IdempotencyTests.cs

@@ -45,6 +45,31 @@ public class IdempotencyTests(TestWebApplicationFactory factory) : IClassFixture
     Assert.NotEqual(firstBody!.Referentie, secondBody!.Referentie);
   }
 
+  // RB-18/BIO-018: IdempotencyStore used to key on the raw client-supplied header alone, so
+  // caller B replaying caller A's Idempotency-Key got caller A's cached reference back —
+  // a cross-caller leak of a value caller B never submitted. The store now keys on
+  // "{SubjectId}:{idemKey}", so the same header value from two different callers is two
+  // independent submissions.
+  [Fact]
+  public async Task A_caller_replaying_another_callers_idempotency_key_does_not_get_their_cached_result()
+  {
+    var sharedKey = Guid.NewGuid().ToString();
+
+    var callerARequest = ChangeRequestWithKey(sharedKey);
+    callerARequest.Headers.Add("X-Subject", "111222333");
+    var callerA = await _client.SendAsync(callerARequest);
+    callerA.EnsureSuccessStatusCode();
+    var callerABody = await callerA.Content.ReadFromJsonAsync<ReferentieResponse>();
+
+    var callerBRequest = ChangeRequestWithKey(sharedKey);
+    callerBRequest.Headers.Add("X-Subject", "999888777");
+    var callerB = await _client.SendAsync(callerBRequest);
+    callerB.EnsureSuccessStatusCode();
+    var callerBBody = await callerB.Content.ReadFromJsonAsync<ReferentieResponse>();
+
+    Assert.NotEqual(callerABody!.Referentie, callerBBody!.Referentie);
+  }
+
   [Fact]
   public async Task A_rejected_submission_replays_the_same_rejection_not_a_retry()
   {