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/internal/config/config.go

@@ -1,24 +1,87 @@
 package config
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
+	"time"
+
+	"gopkg.in/yaml.v3"
+
+	appErrors "nex/backend/pkg/errors"
 )
 
+// Config 应用配置
+type Config struct {
+	Server   ServerConfig   `yaml:"server"`
+	Database DatabaseConfig `yaml:"database"`
+	Log      LogConfig      `yaml:"log"`
+}
+
+// ServerConfig 服务器配置
+type ServerConfig struct {
+	Port         int           `yaml:"port"`
+	ReadTimeout  time.Duration `yaml:"read_timeout"`
+	WriteTimeout time.Duration `yaml:"write_timeout"`
+}
+
+// DatabaseConfig 数据库配置
+type DatabaseConfig struct {
+	Path            string        `yaml:"path"`
+	MaxIdleConns    int           `yaml:"max_idle_conns"`
+	MaxOpenConns    int           `yaml:"max_open_conns"`
+	ConnMaxLifetime time.Duration `yaml:"conn_max_lifetime"`
+}
+
+// LogConfig 日志配置
+type LogConfig struct {
+	Level      string `yaml:"level"`
+	Path       string `yaml:"path"`
+	MaxSize    int    `yaml:"max_size"`
+	MaxBackups int    `yaml:"max_backups"`
+	MaxAge     int    `yaml:"max_age"`
+	Compress   bool   `yaml:"compress"`
+}
+
+// DefaultConfig returns default config values
+func DefaultConfig() *Config {
+	// Use home dir for default paths
+	homeDir, _ := os.UserHomeDir()
+	nexDir := filepath.Join(homeDir, ".nex")
+
+	return &Config{
+		Server: ServerConfig{
+			Port:         9826,
+			ReadTimeout:  30 * time.Second,
+			WriteTimeout: 30 * time.Second,
+		},
+		Database: DatabaseConfig{
+			Path:            filepath.Join(nexDir, "config.db"),
+			MaxIdleConns:    10,
+			MaxOpenConns:    100,
+			ConnMaxLifetime: 1 * time.Hour,
+		},
+		Log: LogConfig{
+			Level:      "info",
+			Path:       filepath.Join(nexDir, "log"),
+			MaxSize:    100,
+			MaxBackups: 10,
+			MaxAge:     30,
+			Compress:   true,
+		},
+	}
+}
+
 // GetConfigDir 获取配置目录路径（~/.nex/）
 func GetConfigDir() (string, error) {
 	homeDir, err := os.UserHomeDir()
 	if err != nil {
 		return "", err
 	}
-
 	configDir := filepath.Join(homeDir, ".nex")
-
-	// 确保目录存在
 	if err := os.MkdirAll(configDir, 0755); err != nil {
 		return "", err
 	}
-
 	return configDir, nil
 }
 
@@ -30,3 +93,79 @@ func GetDBPath() (string, error) {
 	}
 	return filepath.Join(configDir, "config.db"), nil
 }
+
+// GetConfigPath 获取配置文件路径
+func GetConfigPath() (string, error) {
+	configDir, err := GetConfigDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(configDir, "config.yaml"), nil
+}
+
+// LoadConfig loads config from YAML file, creates default if not exists
+func LoadConfig() (*Config, error) {
+	configPath, err := GetConfigPath()
+	if err != nil {
+		return nil, appErrors.Wrap(appErrors.ErrInternal, err)
+	}
+
+	cfg := DefaultConfig()
+
+	data, err := os.ReadFile(configPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			// Create default config file
+			if saveErr := SaveConfig(cfg); saveErr != nil {
+				return nil, appErrors.WithMessage(appErrors.ErrInternal, "创建默认配置失败")
+			}
+			return cfg, nil
+		}
+		return nil, appErrors.Wrap(appErrors.ErrInternal, err)
+	}
+
+	if err := yaml.Unmarshal(data, cfg); err != nil {
+		return nil, appErrors.Wrap(appErrors.ErrInternal, err)
+	}
+
+	return cfg, nil
+}
+
+// SaveConfig saves config to YAML file
+func SaveConfig(cfg *Config) error {
+	configPath, err := GetConfigPath()
+	if err != nil {
+		return appErrors.Wrap(appErrors.ErrInternal, err)
+	}
+
+	data, err := yaml.Marshal(cfg)
+	if err != nil {
+		return appErrors.Wrap(appErrors.ErrInternal, err)
+	}
+
+	// Ensure directory exists
+	dir := filepath.Dir(configPath)
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return appErrors.Wrap(appErrors.ErrInternal, err)
+	}
+
+	return os.WriteFile(configPath, data, 0644)
+}
+
+// Validate validates the config
+func (c *Config) Validate() error {
+	if c.Server.Port < 1 || c.Server.Port > 65535 {
+		return appErrors.WithMessage(appErrors.ErrInvalidRequest, fmt.Sprintf("无效的端口号: %d", c.Server.Port))
+	}
+
+	validLevels := map[string]bool{"debug": true, "info": true, "warn": true, "error": true}
+	if !validLevels[c.Log.Level] {
+		return appErrors.WithMessage(appErrors.ErrInvalidRequest, fmt.Sprintf("无效的日志级别: %s", c.Log.Level))
+	}
+
+	if c.Database.Path == "" {
+		return appErrors.WithMessage(appErrors.ErrInvalidRequest, "数据库路径不能为空")
+	}
+
+	return nil
+}