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>

Program.cs

@@ -49,6 +49,13 @@ builder.Services
     {
         options.SerializerSettings.NullValueHandling = NullValueHandling.Ignore;
 
+        // The SDK validates every DateTime member of a Post/Patch model with a UTC check and
+        // rejects the model when Kind is not Utc. Newtonsoft's default (RoundtripKind) leaves a
+        // date without a zone, e.g. "2026-08-25", as Unspecified, so such a request failed
+        // validation before it ever reached iDoklad. Utc marks the value as UTC without shifting
+        // it and converts values that do carry an offset.
+        options.SerializerSettings.DateTimeZoneHandling = DateTimeZoneHandling.Utc;
+
         // Some IdokladSdk converters attached to model members only support reading (their
         // WriteJson throws NotImplementedException), which makes responses carrying such models
         // fail. The resolver bypasses them on the write path; the existing naming strategy is