test(backend): 补充改动代码单元测试覆盖率至 85%+

新增 48 个测试用例覆盖修复代码的各分支路径：
- subscription_maintenance_queue: nil receiver/task、Stop 幂等、零值参数 (+6)
- billing_service: CalculateCostWithConfig、错误传播、SoraImageCost 等 (+12)
- timing_wheel_service: Schedule/ScheduleRecurring after Stop (+3)
- sora_media_cleanup_service: nil guard、Start/Stop 各分支、timezone (+10)
- sora_gateway_service: normalizeSoraMediaURLs、buildSoraContent 等辅助函数 (+17)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/subscription_maintenance_queue_test.go

@@ -3,6 +3,7 @@
 package service
 
 import (
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -52,3 +53,81 @@ func TestSubscriptionMaintenanceQueue_TryEnqueue_PanicDoesNotKillWorker(t *testi
 		t.Fatalf("worker did not continue after panic")
 	}
 }
+
+func TestSubscriptionMaintenanceQueue_TryEnqueue_AfterStop(t *testing.T) {
+	q := NewSubscriptionMaintenanceQueue(1, 8)
+	q.Stop()
+
+	err := q.TryEnqueue(func() {})
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "stopped")
+}
+
+func TestSubscriptionMaintenanceQueue_TryEnqueue_NilReceiver(t *testing.T) {
+	var q *SubscriptionMaintenanceQueue
+	err := q.TryEnqueue(func() {})
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "nil")
+}
+
+func TestSubscriptionMaintenanceQueue_TryEnqueue_NilTask(t *testing.T) {
+	q := NewSubscriptionMaintenanceQueue(1, 8)
+	t.Cleanup(q.Stop)
+
+	err := q.TryEnqueue(nil)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "nil")
+}
+
+func TestSubscriptionMaintenanceQueue_Stop_NilReceiver(t *testing.T) {
+	var q *SubscriptionMaintenanceQueue
+	// 不应 panic
+	q.Stop()
+}
+
+func TestSubscriptionMaintenanceQueue_Stop_Idempotent(t *testing.T) {
+	q := NewSubscriptionMaintenanceQueue(1, 4)
+	q.Stop()
+	q.Stop() // 第二次调用不应 panic
+}
+
+func TestNewSubscriptionMaintenanceQueue_ZeroParams(t *testing.T) {
+	q := NewSubscriptionMaintenanceQueue(0, 0)
+	t.Cleanup(q.Stop)
+
+	// workerCount/queueSize 应被修正为 1
+	err := q.TryEnqueue(func() {})
+	require.NoError(t, err)
+}
+
+func TestNewSubscriptionMaintenanceQueue_NegativeParams(t *testing.T) {
+	q := NewSubscriptionMaintenanceQueue(-1, -1)
+	t.Cleanup(q.Stop)
+
+	err := q.TryEnqueue(func() {})
+	require.NoError(t, err)
+}
+
+func TestSubscriptionMaintenanceQueue_ConcurrentEnqueueAndStop(t *testing.T) {
+	// 并发调用 TryEnqueue 和 Stop 不应 panic
+	for i := 0; i < 100; i++ {
+		q := NewSubscriptionMaintenanceQueue(2, 4)
+		var wg sync.WaitGroup
+		wg.Add(2)
+
+		go func() {
+			defer wg.Done()
+			for j := 0; j < 50; j++ {
+				_ = q.TryEnqueue(func() {})
+			}
+		}()
+
+		go func() {
+			defer wg.Done()
+			time.Sleep(time.Microsecond * time.Duration(i%10))
+			q.Stop()
+		}()
+
+		wg.Wait()
+	}
+}