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/TestsPage.tsx

@@ -224,8 +224,8 @@ export default function TestsPage() {
           bv = b.created_at || "";
           break;
         case "updated_at":
-          av = a.updated_at || "";
-          bv = b.updated_at || "";
+          av = lastActivityDate(a) || "";
+          bv = lastActivityDate(b) || "";
           break;
         case "waiting_time": {
           const aIsBlue = a.state === "blue_evaluating";
@@ -281,6 +281,23 @@ export default function TestsPage() {
     });
   };
 
+  // updated_at doesn't exist in DB — derive "last activity" from the most
+  // recent non-null timestamp available on the test record
+  const lastActivityDate = (t: import("../types/models").Test): string | null => {
+    const candidates = [
+      t.blue_validated_at,
+      t.red_validated_at,
+      (t as any).blue_work_started_at,
+      (t as any).blue_started_at,
+      (t as any).red_started_at,
+      t.created_at,
+    ].filter(Boolean) as string[];
+    if (!candidates.length) return null;
+    return candidates.reduce((latest, d) =>
+      new Date(d) > new Date(latest) ? d : latest
+    );
+  };
+
   // ── My tasks label ────────────────────────────────────────────────
   const myTasksLabel = useMemo(() => {
     if (!user) return "My Tasks";
@@ -594,7 +611,7 @@ export default function TestsPage() {
                     {formatDate(test.created_at)}
                   </td>
                   <td className="py-3 px-4 text-gray-400 text-xs whitespace-nowrap">
-                    {formatDate(test.updated_at)}
+                    {formatDate(lastActivityDate(test))}
                   </td>
                   <td className="py-3 pl-4">
                     <button