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_test.go

@@ -0,0 +1,176 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"gopkg.in/yaml.v3"
+)
+
+func TestDefaultConfig(t *testing.T) {
+	cfg := DefaultConfig()
+	require.NotNil(t, cfg)
+
+	assert.Equal(t, 9826, cfg.Server.Port)
+	assert.Equal(t, 30*time.Second, cfg.Server.ReadTimeout)
+	assert.Equal(t, 30*time.Second, cfg.Server.WriteTimeout)
+
+	assert.Equal(t, 10, cfg.Database.MaxIdleConns)
+	assert.Equal(t, 100, cfg.Database.MaxOpenConns)
+	assert.Equal(t, 1*time.Hour, cfg.Database.ConnMaxLifetime)
+
+	assert.Equal(t, "info", cfg.Log.Level)
+	assert.Equal(t, 100, cfg.Log.MaxSize)
+	assert.Equal(t, 10, cfg.Log.MaxBackups)
+	assert.Equal(t, 30, cfg.Log.MaxAge)
+	assert.Equal(t, true, cfg.Log.Compress)
+}
+
+func TestConfig_Validate(t *testing.T) {
+	tests := []struct {
+		name    string
+		modify  func(*Config)
+		wantErr bool
+		errMsg  string
+	}{
+		{
+			name:    "默认配置有效",
+			modify:  func(c *Config) {},
+			wantErr: false,
+		},
+		{
+			name:    "端口号为0无效",
+			modify:  func(c *Config) { c.Server.Port = 0 },
+			wantErr: true,
+			errMsg:  "无效的端口号",
+		},
+		{
+			name:    "端口号超出范围无效",
+			modify:  func(c *Config) { c.Server.Port = 70000 },
+			wantErr: true,
+			errMsg:  "无效的端口号",
+		},
+		{
+			name:    "端口号为1有效",
+			modify:  func(c *Config) { c.Server.Port = 1 },
+			wantErr: false,
+		},
+		{
+			name:    "端口号为65535有效",
+			modify:  func(c *Config) { c.Server.Port = 65535 },
+			wantErr: false,
+		},
+		{
+			name:    "无效日志级别",
+			modify:  func(c *Config) { c.Log.Level = "invalid" },
+			wantErr: true,
+			errMsg:  "无效的日志级别",
+		},
+		{
+			name:    "debug级别有效",
+			modify:  func(c *Config) { c.Log.Level = "debug" },
+			wantErr: false,
+		},
+		{
+			name:    "warn级别有效",
+			modify:  func(c *Config) { c.Log.Level = "warn" },
+			wantErr: false,
+		},
+		{
+			name:    "error级别有效",
+			modify:  func(c *Config) { c.Log.Level = "error" },
+			wantErr: false,
+		},
+		{
+			name:    "数据库路径为空无效",
+			modify:  func(c *Config) { c.Database.Path = "" },
+			wantErr: true,
+			errMsg:  "数据库路径不能为空",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cfg := DefaultConfig()
+			tt.modify(cfg)
+			err := cfg.Validate()
+			if tt.wantErr {
+				assert.Error(t, err)
+				assert.Contains(t, err.Error(), tt.errMsg)
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}
+
+func TestGetConfigDir(t *testing.T) {
+	dir, err := GetConfigDir()
+	require.NoError(t, err)
+	assert.NotEmpty(t, dir)
+	assert.Contains(t, dir, ".nex")
+}
+
+func TestGetDBPath(t *testing.T) {
+	path, err := GetDBPath()
+	require.NoError(t, err)
+	assert.NotEmpty(t, path)
+	assert.Contains(t, path, "config.db")
+}
+
+func TestGetConfigPath(t *testing.T) {
+	path, err := GetConfigPath()
+	require.NoError(t, err)
+	assert.NotEmpty(t, path)
+	assert.Contains(t, path, "config.yaml")
+}
+
+func TestSaveAndLoadConfig(t *testing.T) {
+	// 使用临时目录覆盖配置路径
+	dir := t.TempDir()
+
+	cfg := &Config{
+		Server: ServerConfig{
+			Port:         9999,
+			ReadTimeout:  10 * time.Second,
+			WriteTimeout: 20 * time.Second,
+		},
+		Database: DatabaseConfig{
+			Path:            filepath.Join(dir, "test.db"),
+			MaxIdleConns:    5,
+			MaxOpenConns:    50,
+			ConnMaxLifetime: 30 * time.Minute,
+		},
+		Log: LogConfig{
+			Level:      "debug",
+			Path:       filepath.Join(dir, "log"),
+			MaxSize:    50,
+			MaxBackups: 5,
+			MaxAge:     7,
+			Compress:   false,
+		},
+	}
+
+	// 保存配置
+	configPath := filepath.Join(dir, "config.yaml")
+	data, err := yaml.Marshal(cfg)
+	require.NoError(t, err)
+	err = os.WriteFile(configPath, data, 0644)
+	require.NoError(t, err)
+
+	// 加载配置
+	data, err = os.ReadFile(configPath)
+	require.NoError(t, err)
+	loaded := &Config{}
+	err = yaml.Unmarshal(data, loaded)
+	require.NoError(t, err)
+
+	assert.Equal(t, cfg.Server.Port, loaded.Server.Port)
+	assert.Equal(t, cfg.Log.Level, loaded.Log.Level)
+	assert.Equal(t, cfg.Database.MaxIdleConns, loaded.Database.MaxIdleConns)
+	assert.Equal(t, cfg.Log.Compress, loaded.Log.Compress)
+}