fix(site-secrets): keep a live node's mirror-status row from losing a race with the peer's prune

Rig finding on the upgrade restart: both nodes' rows were older than the stale threshold
(the old build never refreshed them), so node-a's reporter pruned node-b's row while node-b
wrote a fresh one, and the replicated delete won. The reporter now prunes nothing during its
first refresh interval after start-up, and each mirror pass re-writes its own row if it no
longer holds what the mirror last wrote.

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Secrets/DeployedSecretsMirrorTests.cs

@@ -715,6 +715,26 @@ public class DeployedSecretsMirrorTests : IDisposable
         Assert.Equal(time.Now, Assert.Single(await storage.GetSecretMirrorStatusesAsync()).CheckedAtUtc);
     }
 
+    [Fact]
+    public async Task Status_RowDeletedByAPeerPrune_IsRewrittenOnTheNextPass()
+    {
+        // Rig finding: when both nodes restart together, the peer's reporter can prune this
+        // node's old row concurrently with this node's fresh write, and the replicated delete
+        // wins. The mirror notices its row is gone and writes it again.
+        var time = new FakeTimeProvider(new DateTimeOffset(2026, 9, 28, 6, 0, 0, TimeSpan.Zero));
+        var (mirror, storage, _, _, _) = await CreateAsync(time: time, nodeIdentity: new FixedNodeIdentity("node-b"));
+        await mirror.ReconcileAsync(CancellationToken.None);
+        var written = Assert.Single(await storage.GetSecretMirrorStatusesAsync()).CheckedAtUtc;
+        Assert.True(await storage.DeleteSecretMirrorStatusIfUnchangedAsync("node-b", written));
+
+        time.Now = time.Now.AddSeconds(15);
+        await mirror.ReconcileAsync(CancellationToken.None);
+
+        var status = Assert.Single(await storage.GetSecretMirrorStatusesAsync());
+        Assert.Equal("node-b", status.NodeName);
+        Assert.Equal(time.Now, status.CheckedAtUtc);
+    }
+
     [Fact]
     public async Task Status_FailureSetChange_RewritesTheRow_AndClearsWhenHealed()
     {