fix(ai): meter synchronous generations

JobTrackerApi/Services/AiUsageExecutionScope.cs

@@ -0,0 +1,25 @@
+namespace JobTrackerApi.Services;
+
+public sealed class AiUsageExecutionScope
+{
+    private readonly AsyncLocal<int> _suppressionDepth = new();
+
+    public bool IsSuppressed => _suppressionDepth.Value > 0;
+
+    public IDisposable Suppress()
+    {
+        _suppressionDepth.Value++;
+        return new Restore(this);
+    }
+
+    private sealed class Restore(AiUsageExecutionScope owner) : IDisposable
+    {
+        private AiUsageExecutionScope? _owner = owner;
+
+        public void Dispose()
+        {
+            var current = Interlocked.Exchange(ref _owner, null);
+            if (current is not null) current._suppressionDepth.Value--;
+        }
+    }
+}