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/Makefile

@@ -1,4 +1,4 @@
-.PHONY: build run test test-coverage clean migrate-up migrate-down migrate-status migrate-create lint
+.PHONY: build run test test-unit test-integration test-coverage clean migrate-up migrate-down migrate-status migrate-create lint generate deps
 
 # 构建
 build:
@@ -12,6 +12,14 @@ run:
 test:
 	go test ./... -v
 
+# 单元测试
+test-unit:
+	go test ./internal/... ./pkg/... -v
+
+# 集成测试
+test-integration:
+	go test ./tests/... -v
+
 # 测试覆盖率
 test-coverage:
 	go test ./... -coverprofile=coverage.out
@@ -38,8 +46,12 @@ migrate-create:
 
 # 代码检查
 lint:
-	golangci-lint run ./...
+	go tool golangci-lint run ./...
 
 # 安装依赖
 deps:
 	go mod tidy
+
+# 生成代码（mock 等）
+generate:
+	go generate ./...