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/pkg/logger/logger_test.go

@@ -0,0 +1,138 @@
+package logger
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/zap"
+)
+
+func TestNew_StdoutOnly(t *testing.T) {
+	logger, err := New(Config{Level: "info"})
+	require.NoError(t, err)
+	require.NotNil(t, logger)
+	assert.NoError(t, logger.Sync())
+}
+
+func TestNew_WithFileOutput(t *testing.T) {
+	dir := filepath.Join(os.TempDir(), "nex-logger-test")
+	os.MkdirAll(dir, 0755)
+	defer os.RemoveAll(dir)
+
+	logger, err := New(Config{
+		Level:      "debug",
+		Path:       dir,
+		MaxSize:    10,
+		MaxBackups: 3,
+		MaxAge:     7,
+		Compress:   false,
+	})
+	require.NoError(t, err)
+	require.NotNil(t, logger)
+
+	logger.Info("test log message")
+	_ = logger.Sync()
+
+	// 验证日志文件已创建
+	files, err := os.ReadDir(dir)
+	require.NoError(t, err)
+	assert.NotEmpty(t, files, "日志目录应包含文件")
+}
+
+func TestNew_AllLevels(t *testing.T) {
+	levels := []string{"debug", "info", "warn", "error"}
+	for _, level := range levels {
+		logger, err := New(Config{Level: level})
+		assert.NoError(t, err, "级别 %s 应有效", level)
+		assert.NotNil(t, logger)
+		assert.NoError(t, logger.Sync())
+	}
+}
+
+func TestNew_EmptyLevel(t *testing.T) {
+	// 空级别应默认为 info
+	logger, err := New(Config{Level: ""})
+	require.NoError(t, err)
+	require.NotNil(t, logger)
+	assert.NoError(t, logger.Sync())
+}
+
+func TestNew_InvalidPath(t *testing.T) {
+	// 不可写的路径
+	logger, err := New(Config{
+		Level: "info",
+		Path:  "/nonexistent/deeply/nested/path/logs",
+	})
+	// 应能创建 logger（错误在写入时发生）
+	// 实际上 lumberjack 会尝试创建目录
+	_ = logger
+	_ = err
+}
+
+func TestParseLevel(t *testing.T) {
+	tests := []struct {
+		input string
+		valid bool
+	}{
+		{"debug", true},
+		{"info", true},
+		{"warn", true},
+		{"error", true},
+		{"", true},    // 默认为 info
+		{"invalid", true}, // 默认为 info
+	}
+	for _, tt := range tests {
+		_, err := parseLevel(tt.input)
+		assert.NoError(t, err, "parseLevel(%q) 不应报错", tt.input)
+	}
+}
+
+func TestLogFilePath(t *testing.T) {
+	result := logFilePath(filepath.Join("var", "log"))
+	assert.Contains(t, result, "nex-")
+	assert.Contains(t, result, ".log")
+}
+
+func TestLogFileName(t *testing.T) {
+	name := logFileName()
+	assert.Contains(t, name, "nex-")
+	assert.Contains(t, name, ".log")
+	assert.Len(t, name, len("nex-2006-01-02.log"))
+}
+
+func TestNewRotateWriter_Defaults(t *testing.T) {
+	cfg := Config{
+		Path:     t.TempDir(),
+		MaxSize:  0,
+		MaxAge:   0,
+		Compress: true,
+	}
+	writer := newRotateWriter(cfg)
+	require.NotNil(t, writer)
+	assert.Equal(t, 100, writer.MaxSize)
+	assert.Equal(t, 10, writer.MaxBackups)
+	assert.Equal(t, 30, writer.MaxAge)
+}
+
+func TestWithRequestID(t *testing.T) {
+	logger, err := New(Config{Level: "info"})
+	require.NoError(t, err)
+
+	contextLogger := WithRequestID(logger, "test-request-123")
+	assert.NotNil(t, contextLogger)
+	assert.IsType(t, &zap.Logger{}, contextLogger)
+}
+
+func TestWithContext(t *testing.T) {
+	logger, err := New(Config{Level: "info"})
+	require.NoError(t, err)
+
+	contextLogger := WithContext(logger, map[string]interface{}{
+		"key1": "value1",
+		"key2": 42,
+	})
+	assert.NotNil(t, contextLogger)
+}