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/authn/store_test.go

@@ -0,0 +1,108 @@
+package authn
+
+import (
+	"errors"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/crypto/bcrypt"
+)
+
+func openTestStore(t *testing.T) (*Store, string) {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), DatabaseFile)
+	store, err := Open(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { _ = store.Close() })
+	return store, path
+}
+
+func TestPasswordRecordPersistsAndAuthenticates(t *testing.T) {
+	store, path := openTestStore(t)
+	created, wasCreated, err := store.SetPassword("Kami", "correct horse battery")
+	if err != nil || !wasCreated || created.Username != "Kami" {
+		t.Fatalf("created=%+v new=%v err=%v", created, wasCreated, err)
+	}
+	if _, err := store.Authenticate("KAMI", "correct horse battery"); err != nil {
+		t.Fatalf("authenticate: %v", err)
+	}
+	if _, err := store.Authenticate("Kami", "wrong password"); !errors.Is(err, ErrInvalidCredentials) {
+		t.Fatalf("wrong password error = %v", err)
+	}
+	if err := store.Close(); err != nil {
+		t.Fatal(err)
+	}
+	reopened, err := Open(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer reopened.Close()
+	if _, err := reopened.Authenticate("kami", "correct horse battery"); err != nil {
+		t.Fatalf("persisted authentication: %v", err)
+	}
+}
+
+func TestUpdateRequiresCurrentPasswordAndMovesUsername(t *testing.T) {
+	store, _ := openTestStore(t)
+	if _, _, err := store.SetPassword("operator", "original password"); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := store.Update("operator", "wrong password", "kami", "replacement password"); !errors.Is(err, ErrInvalidCredentials) {
+		t.Fatalf("wrong current password error = %v", err)
+	}
+	updated, err := store.Update("operator", "original password", "kami", "replacement password")
+	if err != nil || updated.Username != "kami" {
+		t.Fatalf("updated=%+v err=%v", updated, err)
+	}
+	if _, err := store.Authenticate("operator", "original password"); !errors.Is(err, ErrInvalidCredentials) {
+		t.Fatalf("old credential error = %v", err)
+	}
+	if _, err := store.Authenticate("kami", "replacement password"); err != nil {
+		t.Fatalf("new credential: %v", err)
+	}
+}
+
+func TestUpdateRefusesExistingUsername(t *testing.T) {
+	store, _ := openTestStore(t)
+	if _, _, err := store.SetPassword("one", "password one"); err != nil {
+		t.Fatal(err)
+	}
+	if _, _, err := store.SetPassword("two", "password two"); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := store.Update("one", "password one", "TWO", ""); !errors.Is(err, ErrUsernameExists) {
+		t.Fatalf("collision error = %v", err)
+	}
+}
+
+func TestLegacyHashImportsOnlyIntoEmptyDatabase(t *testing.T) {
+	store, _ := openTestStore(t)
+	hash, err := bcrypt.GenerateFromPassword([]byte("legacy password"), bcrypt.MinCost)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if imported, err := store.ImportBcrypt("legacy", string(hash)); err != nil || !imported {
+		t.Fatalf("imported=%v err=%v", imported, err)
+	}
+	if imported, err := store.ImportBcrypt("intruder", string(hash)); err != nil || imported {
+		t.Fatalf("second import=%v err=%v", imported, err)
+	}
+	if _, err := store.Authenticate("legacy", "legacy password"); err != nil {
+		t.Fatalf("imported credential: %v", err)
+	}
+}
+
+func TestCredentialValidation(t *testing.T) {
+	store, _ := openTestStore(t)
+	if _, _, err := store.SetPassword("", "a sufficiently long password"); err == nil {
+		t.Fatal("blank username accepted")
+	}
+	if _, _, err := store.SetPassword("operator", "short"); err == nil {
+		t.Fatal("short password accepted")
+	}
+	if _, _, err := store.SetPassword("operator", string(make([]byte, maximumPassword+1))); err == nil {
+		t.Fatal("oversized bcrypt password accepted")
+	}
+}