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/repository/http_upstream_benchmark_test.go

@@ -0,0 +1,46 @@
+package repository
+
+import (
+	"net/http"
+	"net/url"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+)
+
+var httpClientSink *http.Client
+
+// BenchmarkHTTPUpstreamProxyClient 对比重复创建与复用代理客户端的开销。
+func BenchmarkHTTPUpstreamProxyClient(b *testing.B) {
+	cfg := &config.Config{
+		Gateway: config.GatewayConfig{ResponseHeaderTimeout: 300},
+	}
+	upstream := NewHTTPUpstream(cfg)
+	svc, ok := upstream.(*httpUpstreamService)
+	if !ok {
+		b.Fatalf("类型断言失败，无法获取 httpUpstreamService")
+	}
+
+	proxyURL := "http://127.0.0.1:8080"
+	b.ReportAllocs()
+
+	b.Run("新建", func(b *testing.B) {
+		parsedProxy, err := url.Parse(proxyURL)
+		if err != nil {
+			b.Fatalf("解析代理地址失败: %v", err)
+		}
+		for i := 0; i < b.N; i++ {
+			httpClientSink = &http.Client{
+				Transport: buildUpstreamTransport(cfg, parsedProxy),
+			}
+		}
+	})
+
+	b.Run("复用", func(b *testing.B) {
+		client := svc.getOrCreateClient(proxyURL)
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			httpClientSink = client
+		}
+	})
+}