feat(usage): hold the last good TokenTracker export fail-open

src/ClaudeDo.Worker/Usage/TokenTracker/TokenTrackerState.cs

@@ -0,0 +1,55 @@
+namespace ClaudeDo.Worker.Usage.TokenTracker;
+
+/// <summary>
+/// Threadsafe holder for the last successful export. Mirrors <see cref="UsageState"/>: a failed
+/// fetch never overwrites a good export, it only records <see cref="LastError"/>.
+/// </summary>
+public sealed class TokenTrackerState
+{
+    private readonly object _lock = new();
+    private TokenTrackerExport? _export;
+    private DateTime? _lastAttemptUtc;
+    private string? _lastError;
+
+    public TokenTrackerExport? Export
+    {
+        get { lock (_lock) return _export; }
+    }
+
+    public DateTime? LastAttemptUtc
+    {
+        get { lock (_lock) return _lastAttemptUtc; }
+    }
+
+    public string? LastError
+    {
+        get { lock (_lock) return _lastError; }
+    }
+
+    public void ReportSuccess(TokenTrackerExport export)
+    {
+        lock (_lock)
+        {
+            _export = export;
+            _lastAttemptUtc = export.FetchedAtUtc;
+            _lastError = null;
+        }
+    }
+
+    public void ReportFailure(string error, DateTime attemptedAtUtc)
+    {
+        lock (_lock)
+        {
+            _lastAttemptUtc = attemptedAtUtc;
+            _lastError = error;
+        }
+    }
+
+    public bool IsOlderThan(TimeSpan maxAge, DateTime nowUtc)
+    {
+        lock (_lock)
+        {
+            return _export is null || nowUtc - _export.FetchedAtUtc > maxAge;
+        }
+    }
+}