oprava datumu: prijem data bez zony jako UTC

SDK validuje Post/Patch modely pred odeslanim a u kazde polozky DateTime
vyzaduje Kind == Utc. Datum bez zony, napriklad "2026-08-25", nacetl
Newtonsoft jako Unspecified, takze POST /issued-invoices koncil chybou
"DateTime must be in UTC format" jeste pred volanim iDokladu.

- DateTimeZoneHandling.Utc: hodnota bez zony dostane Kind Utc bez posunu,
  hodnota s offsetem se prepocita do UTC
- ExceptionHandlingMiddleware zachytava IdokladValidationException a vraci
  400 se seznamem vadnych vlastnosti misto prazdne 500

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

Infrastructure/ExceptionHandlingMiddleware.cs

@@ -52,6 +52,16 @@ public sealed class ExceptionHandlingMiddleware
                 ["idokladError"] = ex.AuthenticationError?.Error,
             });
         }
+        catch (IdokladValidationException ex)
+        {
+            // The SDK validates Post/Patch models before sending them. This does not derive from
+            // IdokladBaseException, so without this catch the request ended as an empty 500.
+            _logger.LogWarning("iDoklad model validation failed: {Message}", ex.Message);
+            await WriteProblem(context, HttpStatusCode.BadRequest, ex.Message, new Dictionary<string, object?>
+            {
+                ["invalidProperties"] = ex.InvalidProperties?.Keys.ToArray(),
+            });
+        }
         catch (IdokladBaseException ex)
         {
             // Other SDK-level failures (malformed responses, batch errors, etc.).