revize endpointu: cteni NullableProperty, filtr v UTC, attachments

Kontrola vsech 180 operaci proti falesnemu iDoklad API a round trip
331 modelu SDK v obou smerech.

- SdkNullablePropertyConverter doplnuje cteni NullableProperty<T>.
  Konvertor SDK umi jen zapis, takze PATCH s takovou polozkou koncil
  prazdnou 500 uz pri cteni tela. Tykalo se 29 modelu. Zapis zustava
  na konvertoru SDK, odchozi payload se nemeni.
- Datum uvnitr NullableProperty se normalizuje na UTC stejne jako
  zbytek serializace.
- ListModifiers parsuje datum ve filtru s AdjustToUniversal a
  AssumeUniversal. Filtr se zonou se drive posouval o offset.
- POST /attachments kontroluje FileName a FileBytes, SDK na ne sahalo
  bez kontroly na null a vracelo neosetrenou 500.
- ExceptionHandlingMiddleware ma posledni zachyt, zadny request uz
  nekonci prazdnou 500.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Controllers/IntegrationController.cs

@@ -96,7 +96,23 @@ public sealed class IntegrationController : ControllerBase
     /// <summary>Upload an attachment for a document.</summary>
     [HttpPost("attachments")]
     public async Task<IActionResult> UploadAttachment([FromBody] AttachmentUploadModel model, CancellationToken ct)
-        => Ok(await _service.UploadAttachmentAsync(model, ct));
+    {
+        // AttachmentUploadModel carries no validation attributes and the SDK dereferences the file
+        // name without a null check, so an incomplete model would end as an unhandled 500.
+        if (string.IsNullOrWhiteSpace(model.FileName))
+        {
+            ModelState.AddModelError(nameof(model.FileName), "The FileName field is required.");
+        }
+
+        if (model.FileBytes is null || model.FileBytes.Length == 0)
+        {
+            ModelState.AddModelError(nameof(model.FileBytes), "The FileBytes field is required.");
+        }
+
+        return ModelState.IsValid
+            ? Ok(await _service.UploadAttachmentAsync(model, ct))
+            : ValidationProblem(ModelState);
+    }
 
     /// <summary>Delete a single attachment by its id.</summary>
     [HttpDelete("attachments/{attachmentId:int}")]