Legal holds, step 6: what each hold keeps

inbuxa:LegalHold/get answers accountsCovered, itemsHeld and sizeHeld
when asked: the accounts a hold reaches now (deleted ones it keeps
included) and the archived items it keeps, with their size. Worked out
in one pass over accounts and archive, only for requests that name them.
Held items stay out of the user's quota, as all archived copies do
(LH-9).

tests/src/system/legal_hold.rs

@@ -411,6 +411,17 @@ pub async fn test(test: &mut TestServer) {
         is_held(&archived(frozen.archived_items().await)),
         "test 6, LH-6: the archived item wasn't frozen"
     );
+    // LH-9: what the hold keeps, for the console
+    let (_, response) = admin
+        .hold_call(
+            "inbuxa:LegalHold/get",
+            json!({"ids": [first], "properties": ["accountsCovered", "itemsHeld", "sizeHeld"]}),
+        )
+        .await;
+    let summary = &response["list"][0];
+    assert_eq!(summary["accountsCovered"], 1, "LH-9: {response}");
+    assert_eq!(summary["itemsHeld"], 1, "LH-9: {response}");
+    assert!(summary["sizeHeld"].as_u64().is_some_and(|s| s > 0), "LH-9: {response}");
 
     let (_, response) = frozen
         .hold_call("x:ArchivedItem/set", json!({"destroy": [item_id]}))