refactor: 重构后端架构，采用 Go 风格四层设计模式

## 主要变更

### 🏗️ 架构重构
- 采用简洁的四层架构：API → Service → Repository → Model
- 遵循 Go 语言最佳实践和命名规范
- 实现依赖注入和接口导向设计
- 统一错误处理和响应格式

### 📁 目录结构优化
- 删除重复模块 (application/, domain/, infrastructure/ 等)
- 规范化命名 (使用 Go 风格的 snake_case)
- 清理无关文件 (package.json, node_modules/ 等)
- 新增规范化的测试目录结构

### 📚 文档系统
- 为每个模块创建详细的 CLAUDE.md 指导文件
- 包含开发规范、最佳实践和使用示例
- 支持模块化开发，缩短上下文长度

### 🔧 开发规范
- 统一接口命名规范 (UserServicer, PhotoRepositoryr)
- 标准化错误处理机制
- 完善的测试策略 (单元测试、集成测试、性能测试)
- 规范化的配置管理

### 🗂️ 新增文件
- cmd/server/ - 服务启动入口和配置
- internal/model/ - 数据模型层 (entity, dto, request)
- pkg/ - 共享工具包 (logger, response, validator)
- tests/ - 完整测试结构
- docs/ - API 文档和架构设计
- .gitignore - Git 忽略文件配置

### 🗑️ 清理内容
- 删除 Node.js 相关文件 (package.json, node_modules/)
- 移除重复的架构目录
- 清理临时文件和构建产物
- 删除重复的文档文件

## 影响
- 提高代码可维护性和可扩展性
- 统一开发规范，提升团队协作效率
- 优化项目结构，符合 Go 语言生态标准
- 完善文档体系，降低上手难度

backend/Makefile

@@ -29,9 +29,13 @@ MIGRATION_DIR := migrations
 
 ##@ Development Environment Commands
 
-dev: ## Start development server with hot reload
-	@printf "$(GREEN)🚀 Starting development server...\n$(NC)"
-	@air -c .air.toml || go run $(MAIN_FILE)
+dev: ## Start development server with SQLite database
+	@printf "$(GREEN)🚀 Starting development server with SQLite...\n$(NC)"
+	@go run cmd/server/main_with_db.go
+
+dev-simple: ## Start simple development server (mock data)
+	@printf "$(GREEN)🚀 Starting simple development server...\n$(NC)"
+	@go run cmd/server/simple_main.go
 
 dev-up: ## Start development environment with Docker
 	@printf "$(GREEN)🐳 Starting development environment...\n$(NC)"
@@ -148,6 +152,34 @@ migrate-down: ## Rollback database migrations
 	@migrate -path $(MIGRATION_DIR) -database "$(DB_URL)" down
 	@printf "$(GREEN)✅ Migrations rolled back!\n$(NC)"
 
+##@ Database Commands
+
+db-reset: ## Reset SQLite database (delete and recreate)
+	@printf "$(GREEN)🗄️  Resetting SQLite database...\n$(NC)"
+	@rm -f photography.db
+	@printf "$(GREEN)✅ Database reset! Will be recreated on next startup.\n$(NC)"
+
+db-backup: ## Backup SQLite database
+	@printf "$(GREEN)💾 Backing up SQLite database...\n$(NC)"
+	@cp photography.db photography_backup_$(BUILD_TIME).db
+	@printf "$(GREEN)✅ Database backed up to photography_backup_$(BUILD_TIME).db\n$(NC)"
+
+db-shell: ## Open SQLite database shell
+	@printf "$(GREEN)🐚 Opening SQLite database shell...\n$(NC)"
+	@sqlite3 photography.db
+
+db-status: ## Show database status and table info
+	@printf "$(GREEN)📊 Database status:\n$(NC)"
+	@if [ -f "photography.db" ]; then \
+		printf "$(BLUE)Database file: photography.db ($(shell ls -lh photography.db | awk '{print $$5}'))\\n$(NC)"; \
+		printf "$(BLUE)Tables:\\n$(NC)"; \
+		sqlite3 photography.db ".tables"; \
+		printf "$(BLUE)Row counts:\\n$(NC)"; \
+		sqlite3 photography.db "SELECT 'Users: ' || COUNT(*) FROM users; SELECT 'Photos: ' || COUNT(*) FROM photos; SELECT 'Categories: ' || COUNT(*) FROM categories; SELECT 'Tags: ' || COUNT(*) FROM tags;"; \
+	else \
+		printf "$(RED)❌ Database not found. Run 'make dev' to create it.\\n$(NC)"; \
+	fi
+
 ##@ Help
 
 help: ## Display this help message