refactor: 后端日志系统重构

- 新增模块化日志器（pkg/logger/module.go）
- 新增 GORM 日志适配器
- 统一日志入口，移除所有 zap.L() 全局 logger 调用
- 字段标准化
- 启动阶段使用结构化日志
- 更新所有相关测试

backend/pkg/logger/field.go

@@ -0,0 +1,77 @@
+package logger
+
+import "go.uber.org/zap"
+
+const (
+	FieldRequestID  = "request_id"
+	FieldProviderID = "provider_id"
+	FieldModelName  = "model_name"
+	FieldMethod     = "method"
+	FieldPath       = "path"
+	FieldStatusCode = "status"
+	FieldLatency    = "latency"
+	FieldClientIP   = "client_ip"
+	FieldQuery      = "query"
+	FieldBodySize   = "body_size"
+	FieldSQL        = "sql"
+	FieldRows       = "rows_affected"
+)
+
+func RequestID(id string) zap.Field {
+	return zap.String(FieldRequestID, id)
+}
+
+func ProviderID(id string) zap.Field {
+	return zap.String(FieldProviderID, id)
+}
+
+func ModelName(name string) zap.Field {
+	return zap.String(FieldModelName, name)
+}
+
+func Method(method string) zap.Field {
+	return zap.String(FieldMethod, method)
+}
+
+func Path(path string) zap.Field {
+	return zap.String(FieldPath, path)
+}
+
+func StatusCode(code int) zap.Field {
+	return zap.Int(FieldStatusCode, code)
+}
+
+func Latency(latency interface{}) zap.Field {
+	switch v := latency.(type) {
+	case int64:
+		return zap.Int64(FieldLatency, v)
+	case int:
+		return zap.Int(FieldLatency, v)
+	default:
+		return zap.Any(FieldLatency, latency)
+	}
+}
+
+func ClientIP(ip string) zap.Field {
+	return zap.String(FieldClientIP, ip)
+}
+
+func Query(query string) zap.Field {
+	return zap.String(FieldQuery, query)
+}
+
+func BodySize(size int) zap.Field {
+	return zap.Int(FieldBodySize, size)
+}
+
+func SQL(sql string) zap.Field {
+	return zap.String(FieldSQL, sql)
+}
+
+func Rows(rows int64) zap.Field {
+	return zap.Int64(FieldRows, rows)
+}
+
+func Err(err error) zap.Field {
+	return zap.Error(err)
+}