fix(admin): unified user search across live + deleted, incl. the retention journal

The email/name/external-id search was scoped to one tab and only looked in the live
identities table, so a deleted account (whose credentials moved to retained_identities on
deletion) could not be found by the email/id it held. Now a people search spans live and
deleted accounts in one query (robots only on the Robots tab) and also matches the
retention journal and the retained real name; results carry a 'deleted' badge. Integration
test: a deleted account is found by its held email and external id.

backend/internal/inttest/delete_test.go

@@ -155,10 +155,15 @@ func TestUserListDeletedFilter(t *testing.T) {
 	if err != nil {
 		t.Fatalf("provision live: %v", err)
 	}
-	gone, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Gone", "")
+	goneTg := "tg-" + uuid.NewString()
+	gone, _, err := store.ProvisionTelegram(ctx, goneTg, "en", "", "Gone", "")
 	if err != nil {
 		t.Fatalf("provision gone: %v", err)
 	}
+	goneEmail := "gone-" + uuid.NewString() + "@example.com"
+	if err := store.AttachIdentity(ctx, gone.ID, account.KindEmail, goneEmail, true); err != nil {
+		t.Fatalf("attach gone email: %v", err)
+	}
 	if err := deleter.AnonymizeAndTombstone(ctx, gone.ID); err != nil {
 		t.Fatalf("delete: %v", err)
 	}
@@ -183,6 +188,24 @@ func TestUserListDeletedFilter(t *testing.T) {
 	if listHasID(deleted, live.ID) {
 		t.Error("a live account must not appear in the Deleted list")
 	}
+
+	// A search spans both lists and reaches the retention journal: a deleted account is
+	// still found by the email and external id it held (both moved to retained_identities
+	// on deletion, out of the live identities table).
+	byEmail, err := store.ListUsers(ctx, account.UserFilter{EmailExact: goneEmail}, 5000, 0)
+	if err != nil {
+		t.Fatalf("search by email: %v", err)
+	}
+	if !listHasID(byEmail, gone.ID) {
+		t.Error("a deleted account must be found by the email it held (retention journal)")
+	}
+	byExt, err := store.ListUsers(ctx, account.UserFilter{ExternalIDMask: goneTg}, 5000, 0)
+	if err != nil {
+		t.Fatalf("search by external id: %v", err)
+	}
+	if !listHasID(byExt, gone.ID) {
+		t.Error("a deleted account must be found by the external id it held (retention journal)")
+	}
 }
 
 // TestConfirmCodeClearsGuest: confirming an email on a guest via ConfirmCode promotes it to