fix: gate scheduler task concurrency

internal/taskscheduler/gate.go

@@ -0,0 +1,31 @@
+package taskscheduler
+
+import "context"
+
+// SingleFlightGate keeps at most one fetched task in flight. Release is
+// idempotent so repeated terminal updates cannot increase capacity.
+type SingleFlightGate struct {
+	token chan struct{}
+}
+
+func NewSingleFlightGate() *SingleFlightGate {
+	gate := &SingleFlightGate{token: make(chan struct{}, 1)}
+	gate.token <- struct{}{}
+	return gate
+}
+
+func (g *SingleFlightGate) Acquire(ctx context.Context) error {
+	select {
+	case <-g.token:
+		return nil
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+}
+
+func (g *SingleFlightGate) Release() {
+	select {
+	case g.token <- struct{}{}:
+	default:
+	}
+}