feat(error): add GESCHICHTE_TYPE_MISMATCH error code with i18n (de/en/es)

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

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

@@ -128,6 +128,8 @@ public enum ErrorCode {
     JOURNEY_ITEM_POSITION_CONFLICT,
     /** The journey already has the maximum allowed number of items (100). 400 */
     JOURNEY_AT_CAPACITY,
+    /** The Geschichte is not of type JOURNEY — journey-item operations are not allowed on it. 400 */
+    GESCHICHTE_TYPE_MISMATCH,
 
     // --- Tags ---
     /** A tag with the given ID does not exist. 404 */