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/minimal.go

@@ -0,0 +1,35 @@
+package logger
+
+import (
+	"go.uber.org/zap"
+	"go.uber.org/zap/zapcore"
+)
+
+func NewMinimal() *zap.Logger {
+	encoder := zapcore.NewConsoleEncoder(zapcore.EncoderConfig{
+		TimeKey:        "ts",
+		LevelKey:       "level",
+		NameKey:        "logger",
+		CallerKey:      "caller",
+		FunctionKey:    zapcore.OmitKey,
+		MessageKey:     "msg",
+		StacktraceKey:  "stacktrace",
+		LineEnding:     zapcore.DefaultLineEnding,
+		EncodeLevel:    zapcore.CapitalColorLevelEncoder,
+		EncodeTime:     zapcore.ISO8601TimeEncoder,
+		EncodeDuration: zapcore.StringDurationEncoder,
+		EncodeCaller:   zapcore.ShortCallerEncoder,
+	})
+
+	core := zapcore.NewCore(
+		encoder,
+		zapcore.AddSync(stdoutWriter{}),
+		zapcore.DebugLevel,
+	)
+
+	return zap.New(core, zap.AddCaller(), zap.AddStacktrace(zap.ErrorLevel))
+}
+
+func Upgrade(minimalLogger *zap.Logger, cfg Config) (*zap.Logger, error) {
+	return New(cfg)
+}