feat(openzaak): bounded retry + flagged write divergence (WP-60)

Local aanvraag/document writes and their paired ZGW writes aren't
transactional; a ZGW failure after the local write succeeds used to
diverge silently. ZgwHttpClient now retries transport-shaped failures
(not 500, which can follow a partial commit on the non-idempotent
statussen/rollen POSTs), and a ZGW failure that survives retry sets
Aanvraag.ZgwError plus a zgw:divergence audit row instead of failing
or diverging quietly. No outbox/reconcile job: three request-triggered
write paths don't justify a persisted queue that would also need to
carry citizen PII for the JWT audit claims.

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

backend/src/BigRegister.Api/Zgw/OpenZaakDocumentSource.cs

@@ -4,6 +4,7 @@ using BigRegister.Api.Contracts;
 using BigRegister.Api.Data;
 using BigRegister.Domain.Authorization;
 using BigRegister.Stamdata;
+using Microsoft.Extensions.Logging;
 
 namespace BigRegister.Api.Zgw;
 
@@ -20,7 +21,9 @@ namespace BigRegister.Api.Zgw;
 /// <see cref="OpenZaakZaakSource"/> — creating a document needs write scope on Documenten;
 /// linking one to a zaak needs write scope on Zaken (the zaakinformatieobject resource).
 /// </summary>
-public sealed class OpenZaakDocumentSource(HttpClient http, ZgwTokenProvider tokens, ZgwOptions options) : IDocumentSource
+public sealed class OpenZaakDocumentSource(
+    HttpClient http, ZgwTokenProvider tokens, ZgwOptions options, ILogger<OpenZaakDocumentSource>? log = null)
+    : IDocumentSource
 {
   private readonly ZgwHttpClient zgw = new(http, tokens);
 
@@ -41,37 +44,54 @@ public sealed class OpenZaakDocumentSource(HttpClient http, ZgwTokenProvider tok
       UploadAsync(localId, categoryId, wizardId, fileName, contentType, content, caller)
           .GetAwaiter().GetResult();
 
+  // WP-60: once DocumentStore.Add (below) has committed, the local document is the record of
+  // truth (per the class doc above) — a ZGW failure past that point is caught, logged, and
+  // leaves DrcUrl null rather than throwing. DrcUrl == null is already the meaningful "not
+  // registered in ZGW yet" detector LinkToZaak skips on, so no separate flag column is needed
+  // here the way ApplicationStore.ZgwError is for the zaak side (see openzaak-integration.md's
+  // "Write resilience" section for why the two write paths differ).
   private async Task<UploadResponse> UploadAsync(
       string localId, string categoryId, string wizardId, string fileName, string contentType,
       byte[] content, CallerIdentity caller)
   {
     var doc = DocumentStore.Add(localId, categoryId, wizardId, fileName, contentType, content, caller.Bsn);
 
-    if (!options.InformatieobjecttypeUrls.TryGetValue(categoryId, out var informatieobjecttypeUrl))
-      throw new InvalidOperationException(
-          $"Zgw:InformatieobjecttypeUrls has no entry for category '{categoryId}'.");
+    try
+    {
+      if (!options.InformatieobjecttypeUrls.TryGetValue(categoryId, out var informatieobjecttypeUrl))
+        throw new InvalidOperationException(
+            $"Zgw:InformatieobjecttypeUrls has no entry for category '{categoryId}'.");
 
-    var eio = await zgw.PostAsync<Eio>($"{options.DrcBaseUrl}/enkelvoudiginformatieobjecten", new CreateEioRequest(
-        Bronorganisatie: options.Bronorganisatie,
-        Creatiedatum: DateOnly.FromDateTime(doc.UploadedAt.UtcDateTime),
-        Titel: fileName,
-        Auteur: options.UserRepresentation,
-        Taal: "nld",
-        Formaat: contentType,
-        Bestandsnaam: fileName,
-        Inhoud: Convert.ToBase64String(content),
-        Informatieobjecttype: informatieobjecttypeUrl,
-        Identificatie: doc.DocumentId,
-        Vertrouwelijkheidaanduiding: ConfidentialiteitFor(categoryId)), caller);
+      var eio = await zgw.PostAsync<Eio>($"{options.DrcBaseUrl}/enkelvoudiginformatieobjecten", new CreateEioRequest(
+          Bronorganisatie: options.Bronorganisatie,
+          Creatiedatum: DateOnly.FromDateTime(doc.UploadedAt.UtcDateTime),
+          Titel: fileName,
+          Auteur: options.UserRepresentation,
+          Taal: "nld",
+          Formaat: contentType,
+          Bestandsnaam: fileName,
+          Inhoud: Convert.ToBase64String(content),
+          Informatieobjecttype: informatieobjecttypeUrl,
+          Identificatie: doc.DocumentId,
+          Vertrouwelijkheidaanduiding: ConfidentialiteitFor(categoryId)), caller);
+
+      DocumentStore.SetDrcUrl(doc.DocumentId, eio.Url);
+    }
+    catch (Exception ex)
+    {
+      log?.LogError(ex, "zgw divergence document={DocumentId} category={CategoryId}", doc.DocumentId, categoryId);
+    }
 
-    DocumentStore.SetDrcUrl(doc.DocumentId, eio.Url);
     return new UploadResponse(doc.DocumentId, doc.LocalId);
   }
 
   /// <summary>Local link always happens (dual-write, same reasoning as upload); additionally,
   /// once a zaak exists, POST a zaakinformatieobject for every document that has a DRC url —
   /// documents uploaded before Zgw:Enabled was ever true (or under a config gap) simply have
-  /// no DrcUrl yet and are skipped, matching "nothing extra to link" for the local case.</summary>
+  /// no DrcUrl yet and are skipped, matching "nothing extra to link" for the local case.
+  /// WP-60: unlike Upload, a ZGW failure here still throws — DocumentStore.Link (the local
+  /// half) already ran above, so the caller (Program.cs's submit endpoint) catching this and
+  /// recording it as a flagged divergence is what closes the gap, not a try/catch in here.</summary>
   public void LinkToZaak(IReadOnlyList<string> documentIds, string? zaakUrl, CallerIdentity caller)
   {
     DocumentStore.Link(documentIds);