fix(review): harden payment, oauth, and migration paths

backend/internal/service/payment_fulfillment_test.go

@@ -5,6 +5,7 @@ package service
 import (
 	"context"
 	"errors"
+	"math"
 	"testing"
 
 	dbent "github.com/Wei-Shaw/sub2api/ent"
@@ -322,6 +323,16 @@ func TestParseLegacyPaymentOrderID(t *testing.T) {
 	assert.False(t, ok)
 }
 
+func TestIsValidProviderAmount(t *testing.T) {
+	t.Parallel()
+
+	assert.True(t, isValidProviderAmount(0.01))
+	assert.False(t, isValidProviderAmount(0))
+	assert.False(t, isValidProviderAmount(-1))
+	assert.False(t, isValidProviderAmount(math.NaN()))
+	assert.False(t, isValidProviderAmount(math.Inf(1)))
+}
+
 func TestValidateProviderNotificationMetadataRejectsAlipaySnapshotMismatch(t *testing.T) {
 	t.Parallel()