fix(stability): 修复慢查询雪崩隐患、系统配置死锁与提号翻页错误

- 消除 system_configs 写锁争抢：使用读写锁与启动预热将默认配置初始化收敛为单例，杜绝读请求开事务导致的 Lock wait timeout exceeded
- 强化财务看板缓存：将 Redis 缓存有效期由 1 分钟延长为 5 分钟，接入 singleflight.Group 防并发击穿
- 修复提号列表 GORM 翻页 Bug：分离 Count 查询与列表查询构建，避免重复 Join 导致的 Error 1066
- 生产环境资源加固：docker-compose.prod.yml 中 MySQL 资源限制提升至 2.0 核 CPU、2G 内存
- 补充 systemconfig 并发测试与 pickup 分页 Count 回归测试

backend/internal/modules/adminfinance/dashboard_test.go

@@ -1,6 +1,8 @@
 package adminfinance
 
 import (
+	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -369,3 +371,33 @@ func TestSummaryFromDailyRowsPreservesDashboardTotals(t *testing.T) {
 		t.Fatalf("订单状态汇总错误: %+v", summary)
 	}
 }
+
+func TestDashboardCacheKeyAndSingleflight(t *testing.T) {
+	repo := NewRepository(nil)
+	q := DashboardQuery{
+		StartDate: time.Date(2026, 9, 1, 0, 0, 0, 0, time.UTC),
+		EndDate:   time.Date(2026, 9, 7, 0, 0, 0, 0, time.UTC),
+	}
+	key := repo.dashboardCacheKey(q)
+	if key == "" {
+		t.Fatal("cache key should not be empty")
+	}
+
+	var callCount atomic.Int32
+	var wg sync.WaitGroup
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			_, _, _ = repo.sfGroup.Do(key, func() (any, error) {
+				callCount.Add(1)
+				time.Sleep(20 * time.Millisecond)
+				return &DashboardDTO{}, nil
+			})
+		}()
+	}
+	wg.Wait()
+	if count := callCount.Load(); count != 1 {
+		t.Fatalf("singleflight 并发合并失败: got %d, want 1", count)
+	}
+}