Bound pending flow states

The login route is unauthenticated, so its pending states could grow
without limit and every Add scanned them all. Expired states now leave
from the front of an add-ordered queue, and Add refuses a new flow with
ErrTooManyPending once 10000 are pending; login answers 503. Both flows
record the state before setting the cookie.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/auth/auth_test.go

@@ -872,6 +872,43 @@ func TestLoginCookieExpires(t *testing.T) {
 	}
 }
 
+func TestPendingStates(t *testing.T) {
+	t.Parallel()
+	p := NewPendingStates(time.Minute)
+	now := time.Now()
+	if err := p.Add("first", now); err != nil {
+		t.Fatal(err)
+	}
+	if !p.Consume("first") {
+		t.Error("Consume of a pending state = false")
+	}
+	if p.Consume("first") {
+		t.Error("second Consume of a state = true")
+	}
+	if p.Consume("never-added") {
+		t.Error("Consume of an unknown state = true")
+	}
+
+	for i := range maxPendingStates {
+		if err := p.Add(fmt.Sprint("s", i), now); err != nil {
+			t.Fatalf("Add %d: %v", i, err)
+		}
+	}
+	if err := p.Add("over", now); !errors.Is(err, ErrTooManyPending) {
+		t.Errorf("Add past the bound = %v, want ErrTooManyPending", err)
+	}
+	// Once the earlier states expire, they are forgotten and a new one fits.
+	if err := p.Add("later", now.Add(time.Minute)); err != nil {
+		t.Errorf("Add after expiry: %v", err)
+	}
+	if p.Consume("s0") {
+		t.Error("an expired state was still pending")
+	}
+	if !p.Consume("later") {
+		t.Error("the state added after expiry is not pending")
+	}
+}
+
 func TestLoginRejectsOffOriginReturn(t *testing.T) {
 	t.Parallel()
 	h := newHarness(t)