fix(tests): replace updated_at (doesn't exist) with real timestamps

TestsPage 'Updated' column: compute lastActivityDate() from the most
recent available timestamp — blue_validated_at > red_validated_at >
blue_work_started_at > blue_started_at > red_started_at > created_at.
Also fixes the sort-by-updated_at case.

ValidatedTestsPage 'Validated' column: use blue_validated_at (when Blue
Lead approved) falling back to red_validated_at. Fixes both the display
and the default sort-by-validated.

frontend/src/pages/ValidatedTestsPage.tsx

@@ -72,7 +72,7 @@ export default function ValidatedTestsPage() {
     | "attack_success"
     | "detection_result"
     | "created_at"
-    | "updated_at";
+    | "updated_at";  // maps to blue_validated_at
 
   const [sortKey, setSortKey] = useState<SortKey>("updated_at");
   const [sortDir, setSortDir] = useState<"asc" | "desc">("desc");
@@ -145,8 +145,9 @@ export default function ValidatedTestsPage() {
           bv = b.created_at || "";
           break;
         case "updated_at":
-          av = a.updated_at || "";
-          bv = b.updated_at || "";
+          // "Validated" column = when Blue Lead validated (last validation step)
+          av = a.blue_validated_at || a.red_validated_at || a.created_at || "";
+          bv = b.blue_validated_at || b.red_validated_at || b.created_at || "";
           break;
       }
       const cmp = av < bv ? -1 : av > bv ? 1 : 0;
@@ -280,7 +281,7 @@ export default function ValidatedTestsPage() {
                     {formatDate(test.created_at)}
                   </td>
                   <td className="px-4 py-3 text-xs text-gray-400 whitespace-nowrap">
-                    {formatDate(test.updated_at)}
+                    {formatDate(test.blue_validated_at || test.red_validated_at)}
                   </td>
                   <td className="px-4 py-3">
                     <button