feat(error): add JOURNEY_AT_CAPACITY 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

@@ -126,6 +126,8 @@ public enum ErrorCode {
     JOURNEY_ITEM_NOT_FOUND,
     /** A position uniqueness conflict occurred on the journey_items table — concurrent append or reorder. 409 */
     JOURNEY_ITEM_POSITION_CONFLICT,
+    /** The journey already has the maximum allowed number of items (100). 400 */
+    JOURNEY_AT_CAPACITY,
 
     // --- Tags ---
     /** A tag with the given ID does not exist. 404 */