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/migrations/004_create_categories.sql

@@ -0,0 +1,113 @@
+-- +migrate Up
+
+CREATE TABLE categories (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100) NOT NULL,
+    description TEXT,
+    slug VARCHAR(255) UNIQUE,
+    parent_id INTEGER REFERENCES categories(id) ON DELETE CASCADE,
+    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    cover_photo_id INTEGER REFERENCES photos(id) ON DELETE SET NULL,
+    color VARCHAR(7) DEFAULT '#3B82F6',
+    is_public BOOLEAN DEFAULT true,
+    photo_count INTEGER DEFAULT 0,
+    album_count INTEGER DEFAULT 0,
+    sort_order INTEGER DEFAULT 0,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    deleted_at TIMESTAMP
+);
+
+-- 创建索引
+CREATE INDEX idx_categories_user_id ON categories(user_id);
+CREATE INDEX idx_categories_parent_id ON categories(parent_id);
+CREATE INDEX idx_categories_cover_photo_id ON categories(cover_photo_id);
+CREATE INDEX idx_categories_slug ON categories(slug);
+CREATE INDEX idx_categories_is_public ON categories(is_public);
+CREATE INDEX idx_categories_created_at ON categories(created_at);
+CREATE INDEX idx_categories_photo_count ON categories(photo_count);
+CREATE INDEX idx_categories_album_count ON categories(album_count);
+CREATE INDEX idx_categories_sort_order ON categories(sort_order);
+CREATE INDEX idx_categories_deleted_at ON categories(deleted_at) WHERE deleted_at IS NOT NULL;
+
+-- 树形结构查询优化
+CREATE INDEX idx_categories_parent_sort ON categories(parent_id, sort_order);
+
+-- 添加更新时间触发器
+CREATE TRIGGER update_categories_updated_at BEFORE UPDATE ON categories
+FOR EACH ROW EXECUTE FUNCTION update_updated_at_column();
+
+-- 添加 slug 自动生成触发器
+CREATE OR REPLACE FUNCTION generate_category_slug()
+RETURNS TRIGGER AS $$
+BEGIN
+    IF NEW.slug IS NULL OR NEW.slug = '' THEN
+        NEW.slug = lower(regexp_replace(NEW.name, '[^a-zA-Z0-9]+', '-', 'g'));
+        NEW.slug = trim(both '-' from NEW.slug);
+        
+        -- 确保 slug 唯一
+        DECLARE
+            counter INTEGER := 0;
+            base_slug VARCHAR(255);
+        BEGIN
+            base_slug := NEW.slug;
+            WHILE EXISTS (SELECT 1 FROM categories WHERE slug = NEW.slug AND id != COALESCE(NEW.id, 0)) LOOP
+                counter := counter + 1;
+                NEW.slug := base_slug || '-' || counter;
+            END LOOP;
+        END;
+    END IF;
+    RETURN NEW;
+END;
+$$ language 'plpgsql';
+
+CREATE TRIGGER generate_categories_slug BEFORE INSERT OR UPDATE ON categories
+FOR EACH ROW EXECUTE FUNCTION generate_category_slug();
+
+-- 插入默认分类
+INSERT INTO categories (name, description, slug, user_id, color, is_public) 
+SELECT 
+    '风景摄影' as name,
+    '自然风光、城市景观等风景类摄影作品' as description,
+    'landscape' as slug,
+    1 as user_id,
+    '#10B981' as color,
+    true as is_public
+WHERE EXISTS (SELECT 1 FROM users WHERE id = 1);
+
+INSERT INTO categories (name, description, slug, user_id, color, is_public) 
+SELECT 
+    '人像摄影' as name,
+    '肖像、写真、人物摄影作品' as description,
+    'portrait' as slug,
+    1 as user_id,
+    '#F59E0B' as color,
+    true as is_public
+WHERE EXISTS (SELECT 1 FROM users WHERE id = 1);
+
+INSERT INTO categories (name, description, slug, user_id, color, is_public) 
+SELECT 
+    '街拍摄影' as name,
+    '街头摄影、日常生活记录' as description,
+    'street' as slug,
+    1 as user_id,
+    '#EF4444' as color,
+    true as is_public
+WHERE EXISTS (SELECT 1 FROM users WHERE id = 1);
+
+INSERT INTO categories (name, description, slug, user_id, color, is_public) 
+SELECT 
+    '建筑摄影' as name,
+    '建筑、室内设计等建筑类摄影作品' as description,
+    'architecture' as slug,
+    1 as user_id,
+    '#8B5CF6' as color,
+    true as is_public
+WHERE EXISTS (SELECT 1 FROM users WHERE id = 1);
+
+-- +migrate Down
+
+DROP TRIGGER IF EXISTS generate_categories_slug ON categories;
+DROP FUNCTION IF EXISTS generate_category_slug();
+DROP TRIGGER IF EXISTS update_categories_updated_at ON categories;
+DROP TABLE IF EXISTS categories;