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/handler/request_body_limit_test.go

@@ -0,0 +1,45 @@
+package handler
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/server/middleware"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRequestBodyLimitTooLarge(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	limit := int64(16)
+	router := gin.New()
+	router.Use(middleware.RequestBodyLimit(limit))
+	router.POST("/test", func(c *gin.Context) {
+		_, err := io.ReadAll(c.Request.Body)
+		if err != nil {
+			if maxErr, ok := extractMaxBytesError(err); ok {
+				c.JSON(http.StatusRequestEntityTooLarge, gin.H{
+					"error": buildBodyTooLargeMessage(maxErr.Limit),
+				})
+				return
+			}
+			c.JSON(http.StatusBadRequest, gin.H{
+				"error": "read_failed",
+			})
+			return
+		}
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+
+	payload := bytes.Repeat([]byte("a"), int(limit+1))
+	req := httptest.NewRequest(http.MethodPost, "/test", bytes.NewReader(payload))
+	recorder := httptest.NewRecorder()
+	router.ServeHTTP(recorder, req)
+
+	require.Equal(t, http.StatusRequestEntityTooLarge, recorder.Code)
+	require.Contains(t, recorder.Body.String(), buildBodyTooLargeMessage(limit))
+}