feat: 系统性改进后端测试体系

- 新增 6 个测试场景 (config load pipe, handler errors, service aggregation, engine degradation, openai decoder edges, negative tests)
- 更新测试工具规格 (mockgen, in-memory SQLite)
- 覆盖率目标从 >80% 提升至 >85%
- 新增 test-unit 和 test-integration Makefile 命令
- 新增死代码清理和 mockgen 需求
- 归档变更至 openspec/changes/archive/2026-04-22-improve-backend-testing/

backend/internal/service/stats_service.go

@@ -6,6 +6,8 @@ import (
 	"nex/backend/internal/domain"
 )
 
+//go:generate go run go.uber.org/mock/mockgen -source=stats_service.go -destination=../../tests/mocks/mock_stats_service.go -package=mocks
+
 // StatsService 统计服务接口
 type StatsService interface {
 	Record(providerID, modelName string) error