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/concurrency_cache_benchmark_test.go

@@ -0,0 +1,135 @@
+package repository
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/redis/go-redis/v9"
+)
+
+// 基准测试用 TTL 配置
+const benchSlotTTLMinutes = 15
+
+var benchSlotTTL = time.Duration(benchSlotTTLMinutes) * time.Minute
+
+// BenchmarkAccountConcurrency 用于对比 SCAN 与有序集合的计数性能。
+func BenchmarkAccountConcurrency(b *testing.B) {
+	rdb := newBenchmarkRedisClient(b)
+	defer func() {
+		_ = rdb.Close()
+	}()
+
+	cache := NewConcurrencyCache(rdb, benchSlotTTLMinutes).(*concurrencyCache)
+	ctx := context.Background()
+
+	for _, size := range []int{10, 100, 1000} {
+		size := size
+		b.Run(fmt.Sprintf("zset/slots=%d", size), func(b *testing.B) {
+			accountID := time.Now().UnixNano()
+			key := accountSlotKey(accountID)
+
+			b.StopTimer()
+			members := make([]redis.Z, 0, size)
+			now := float64(time.Now().Unix())
+			for i := 0; i < size; i++ {
+				members = append(members, redis.Z{
+					Score:  now,
+					Member: fmt.Sprintf("req_%d", i),
+				})
+			}
+			if err := rdb.ZAdd(ctx, key, members...).Err(); err != nil {
+				b.Fatalf("初始化有序集合失败: %v", err)
+			}
+			if err := rdb.Expire(ctx, key, benchSlotTTL).Err(); err != nil {
+				b.Fatalf("设置有序集合 TTL 失败: %v", err)
+			}
+			b.StartTimer()
+
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				if _, err := cache.GetAccountConcurrency(ctx, accountID); err != nil {
+					b.Fatalf("获取并发数量失败: %v", err)
+				}
+			}
+
+			b.StopTimer()
+			if err := rdb.Del(ctx, key).Err(); err != nil {
+				b.Fatalf("清理有序集合失败: %v", err)
+			}
+		})
+
+		b.Run(fmt.Sprintf("scan/slots=%d", size), func(b *testing.B) {
+			accountID := time.Now().UnixNano()
+			pattern := fmt.Sprintf("%s%d:*", accountSlotKeyPrefix, accountID)
+			keys := make([]string, 0, size)
+
+			b.StopTimer()
+			pipe := rdb.Pipeline()
+			for i := 0; i < size; i++ {
+				key := fmt.Sprintf("%s%d:req_%d", accountSlotKeyPrefix, accountID, i)
+				keys = append(keys, key)
+				pipe.Set(ctx, key, "1", benchSlotTTL)
+			}
+			if _, err := pipe.Exec(ctx); err != nil {
+				b.Fatalf("初始化扫描键失败: %v", err)
+			}
+			b.StartTimer()
+
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				if _, err := scanSlotCount(ctx, rdb, pattern); err != nil {
+					b.Fatalf("SCAN 计数失败: %v", err)
+				}
+			}
+
+			b.StopTimer()
+			if err := rdb.Del(ctx, keys...).Err(); err != nil {
+				b.Fatalf("清理扫描键失败: %v", err)
+			}
+		})
+	}
+}
+
+func scanSlotCount(ctx context.Context, rdb *redis.Client, pattern string) (int, error) {
+	var cursor uint64
+	count := 0
+	for {
+		keys, nextCursor, err := rdb.Scan(ctx, cursor, pattern, 100).Result()
+		if err != nil {
+			return 0, err
+		}
+		count += len(keys)
+		if nextCursor == 0 {
+			break
+		}
+		cursor = nextCursor
+	}
+	return count, nil
+}
+
+func newBenchmarkRedisClient(b *testing.B) *redis.Client {
+	b.Helper()
+
+	redisURL := os.Getenv("TEST_REDIS_URL")
+	if redisURL == "" {
+		b.Skip("未设置 TEST_REDIS_URL，跳过 Redis 基准测试")
+	}
+
+	opt, err := redis.ParseURL(redisURL)
+	if err != nil {
+		b.Fatalf("解析 TEST_REDIS_URL 失败: %v", err)
+	}
+
+	client := redis.NewClient(opt)
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	if err := client.Ping(ctx).Err(); err != nil {
+		b.Fatalf("Redis 连接失败: %v", err)
+	}
+
+	return client
+}