perf(后端): 完成性能优化与连接池配置

新增 DB/Redis 连接池配置与校验，并补充单测

网关请求体大小限制与 413 处理

HTTP/req 客户端池化并调整上游连接池默认值

并发槽位改为 ZSET+Lua 与指数退避

用量统计改 SQL 聚合并新增索引迁移

计费缓存写入改工作池并补测试/基准

测试: 在 backend/ 下运行 go test ./...

backend/internal/service/billing_cache_service_test.go

@@ -0,0 +1,75 @@
+package service
+
+import (
+	"context"
+	"errors"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/stretchr/testify/require"
+)
+
+type billingCacheWorkerStub struct {
+	balanceUpdates      int64
+	subscriptionUpdates int64
+}
+
+func (b *billingCacheWorkerStub) GetUserBalance(ctx context.Context, userID int64) (float64, error) {
+	return 0, errors.New("not implemented")
+}
+
+func (b *billingCacheWorkerStub) SetUserBalance(ctx context.Context, userID int64, balance float64) error {
+	atomic.AddInt64(&b.balanceUpdates, 1)
+	return nil
+}
+
+func (b *billingCacheWorkerStub) DeductUserBalance(ctx context.Context, userID int64, amount float64) error {
+	atomic.AddInt64(&b.balanceUpdates, 1)
+	return nil
+}
+
+func (b *billingCacheWorkerStub) InvalidateUserBalance(ctx context.Context, userID int64) error {
+	return nil
+}
+
+func (b *billingCacheWorkerStub) GetSubscriptionCache(ctx context.Context, userID, groupID int64) (*SubscriptionCacheData, error) {
+	return nil, errors.New("not implemented")
+}
+
+func (b *billingCacheWorkerStub) SetSubscriptionCache(ctx context.Context, userID, groupID int64, data *SubscriptionCacheData) error {
+	atomic.AddInt64(&b.subscriptionUpdates, 1)
+	return nil
+}
+
+func (b *billingCacheWorkerStub) UpdateSubscriptionUsage(ctx context.Context, userID, groupID int64, cost float64) error {
+	atomic.AddInt64(&b.subscriptionUpdates, 1)
+	return nil
+}
+
+func (b *billingCacheWorkerStub) InvalidateSubscriptionCache(ctx context.Context, userID, groupID int64) error {
+	return nil
+}
+
+func TestBillingCacheServiceQueueHighLoad(t *testing.T) {
+	cache := &billingCacheWorkerStub{}
+	svc := NewBillingCacheService(cache, nil, nil, &config.Config{})
+	t.Cleanup(svc.Stop)
+
+	start := time.Now()
+	for i := 0; i < cacheWriteBufferSize*2; i++ {
+		svc.QueueDeductBalance(1, 1)
+	}
+	require.Less(t, time.Since(start), 2*time.Second)
+
+	svc.QueueUpdateSubscriptionUsage(1, 2, 1.5)
+
+	require.Eventually(t, func() bool {
+		return atomic.LoadInt64(&cache.balanceUpdates) > 0
+	}, 2*time.Second, 10*time.Millisecond)
+
+	require.Eventually(t, func() bool {
+		return atomic.LoadInt64(&cache.subscriptionUpdates) > 0
+	}, 2*time.Second, 10*time.Millisecond)
+}