feat: 实现分层架构，包含 domain、service、repository 和 pkg 层

- 新增 domain 层：model、provider、route、stats 实体
- 新增 service 层：models、providers、routing、stats 业务逻辑
- 新增 repository 层：models、providers、stats 数据访问
- 新增 pkg 工具包：errors、logger、validator
- 新增中间件：CORS、logging、recovery、request ID
- 新增数据库迁移：初始 schema 和索引
- 新增单元测试和集成测试
- 新增规范文档：config-management、database-migration、error-handling、layered-architecture、middleware-system、request-validation、structured-logging、test-coverage
- 移除 config 子包和 model_router（已迁移至分层架构）

backend/Makefile

@@ -0,0 +1,45 @@
+.PHONY: build run test test-coverage clean migrate-up migrate-down migrate-status migrate-create lint
+
+# 构建
+build:
+	go build -o bin/server ./cmd/server
+
+# 运行
+run:
+	go run ./cmd/server
+
+# 测试
+test:
+	go test ./... -v
+
+# 测试覆盖率
+test-coverage:
+	go test ./... -coverprofile=coverage.out
+	go tool cover -html=coverage.out -o coverage.html
+	@echo "Coverage report generated: coverage.html"
+
+# 清理
+clean:
+	rm -rf bin/ coverage.out coverage.html
+
+# 数据库迁移
+migrate-up:
+	goose -dir migrations sqlite3 $(DB_PATH) up
+
+migrate-down:
+	goose -dir migrations sqlite3 $(DB_PATH) down
+
+migrate-status:
+	goose -dir migrations sqlite3 $(DB_PATH) status
+
+migrate-create:
+	@read -p "Migration name: " name; \
+	goose -dir migrations create $$name sql
+
+# 代码检查
+lint:
+	golangci-lint run ./...
+
+# 安装依赖
+deps:
+	go mod tidy