docs(api): document reorder full-list contract in OpenAPI

Add @Operation annotation to reorderItems() clarifying that itemIds must
contain ALL item IDs for the journey in the desired order — a partial list
returns 400 Bad Request. This surfaces the contract in the generated
OpenAPI spec and Swagger UI.

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

backend/src/main/java/org/raddatz/familienarchiv/geschichte/GeschichteController.java

@@ -102,6 +102,10 @@ public class GeschichteController {
 
     @PutMapping("/{id}/items/reorder")
     @RequirePermission(Permission.BLOG_WRITE)
+    @Operation(
+        summary = "Reorder journey items",
+        description = "itemIds must contain ALL item IDs for the given journey in the desired new order. Sending a partial list returns 400 Bad Request."
+    )
     public List<JourneyItemView> reorderItems(
             @PathVariable UUID id,
             @RequestBody JourneyReorderDTO dto) {