feat(persons): add INVALID_PERSON_TYPE error code with i18n translations

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java

@@ -13,6 +13,8 @@ public enum ErrorCode {
     PERSON_NOT_FOUND,
     /** A person name alias with the given ID does not exist. 404 */
     ALIAS_NOT_FOUND,
+    /** The submitted personType value is not allowed (e.g. SKIP is import-only). 400 */
+    INVALID_PERSON_TYPE,
 
     // --- Documents ---
     /** A document with the given ID does not exist. 404 */