feat: add structured logging system for Docker deployment

添加日志系统，支持 Docker logs 捕获：
- 新增 logger 模块，提供 Info/Warn/Error/Debug 等日志级别
- 任务生命周期日志：创建、开始、完成、失败、取消
- 用户登录/登出日志
- 服务启动和 FFmpeg 状态日志
- 数据库初始化日志

日志格式：[时间] [级别] 消息，便于 Docker logs 查看

Co-Authored-By: AI

internal/router/login.go

@@ -5,6 +5,7 @@ import (
 	"net/http"
 
 	"bilidown/internal/bilibili"
+	"bilidown/internal/logger"
 	"bilidown/internal/util"
 
 	"github.com/skip2/go-qrcode"
@@ -87,6 +88,7 @@ func getQRStatus(w http.ResponseWriter, r *http.Request) {
 		util.Res{Success: false, Message: err.Error()}.Write(w)
 		return
 	}
+	logger.UserLogin()
 	util.Res{Success: true, Message: "登录成功"}.Write(w)
 }
 
@@ -98,5 +100,6 @@ func logout(w http.ResponseWriter, r *http.Request) {
 		util.Res{Success: false, Message: err.Error()}.Write(w)
 		return
 	}
+	logger.UserLogout()
 	util.Res{Success: true, Message: "退出成功"}.Write(w)
 }