Report queued tasks the scheduling pass never considers

A task in retry backoff was filtered out before the candidate loop, so it
recorded no rejection at all: queued, apparently assignable, and silent. That is
the exact shape that made F5 take a live session to diagnose. It now reports
"retry backoff until <time>".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/router/router.go

@@ -182,9 +182,17 @@ func (r *Router) AssignPending() ([]domain.Event, error) {
 	snapshot := r.Store.SchedulingSnapshot()
 	var queued []domain.Task
 	for _, t := range snapshot.Tasks {
-		if t.State == domain.StateQueued && (t.NextRetryAt.IsZero() || !now.Before(t.NextRetryAt)) {
-			queued = append(queued, t)
+		if t.State != domain.StateQueued {
+			continue
 		}
+		if !t.NextRetryAt.IsZero() && now.Before(t.NextRetryAt) {
+			// A queued task the pass never even considers is the most
+			// confusing state of all: it looks assignable and nothing is
+			// recorded against it. Say so.
+			r.reject(t.ID, "", "retry backoff until "+t.NextRetryAt.UTC().Format(time.RFC3339))
+			continue
+		}
+		queued = append(queued, t)
 	}
 	sort.SliceStable(queued, func(i, j int) bool { return importance(queued[i], now).Before(importance(queued[j], now)) })
 	health := r.healthSnapshot(now)

internal/router/router_test.go

@@ -364,6 +364,25 @@ func TestAssignPendingRecordsWhyItPlacedNothing(t *testing.T) {
 		t.Fatalf("pre-lease refusal not reported: %+v", rt.Rejections())
 	}
 
+	// A queued task in retry backoff is skipped before the candidate loop, so
+	// it needs its own reason: it looks assignable and nothing else records it.
+	if err := s.Append(domain.Event{ID: "backoff", TaskID: "t", Type: "TaskCorrected", Version: 2, Payload: backoffPayload(time.Now().Add(time.Hour)), Surface: string(authz.System)}); err == nil {
+		if got, _ := s.Task("t"); !got.NextRetryAt.IsZero() {
+			if _, err := rt.AssignPending(); err != nil {
+				t.Fatal(err)
+			}
+			backoff := false
+			for _, rej := range rt.Rejections() {
+				if strings.Contains(rej.Reason, "retry backoff") {
+					backoff = true
+				}
+			}
+			if !backoff {
+				t.Fatalf("a task in retry backoff was silently skipped: %+v", rt.Rejections())
+			}
+		}
+	}
+
 	// A successful pass leaves nothing behind to misread.
 	s.PreLease = nil
 	if got, err := rt.AssignPending(); err != nil || len(got) != 1 {
@@ -373,3 +392,8 @@ func TestAssignPendingRecordsWhyItPlacedNothing(t *testing.T) {
 		t.Fatalf("stale rejections after a successful pass: %+v", reasons)
 	}
 }
+
+func backoffPayload(at time.Time) []byte {
+	b, _ := json.Marshal(map[string]any{"next_retry_at": at.UTC().Format(time.RFC3339Nano)})
+	return b
+}