fix(admin): address PR #623 review feedback

- Add load() unit tests for admin/users/[id] (permission gate, 404, success)
- Rename .test.ts → .spec.ts for consistency with rest of suite
- Add @Schema(requiredMode=REQUIRED) to InviteListItem.shareableUrl
- Add client-side allowlist for invite status query param

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

backend/src/main/java/org/raddatz/familienarchiv/user/InviteListItemDTO.java

@@ -31,5 +31,6 @@ public class InviteListItemDTO {
     private String status;
     @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private LocalDateTime createdAt;
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private String shareableUrl;
 }