fix: resolve critical issues and improve task management

- Fix path traversal vulnerability in downloadVideo handler by adding
  download directory whitelist validation
- Add graceful shutdown with signal handling for task persistence
- Fix division by zero panic in progressBar.percent() when total <= 0
- Add GetDB() function that returns error instead of using log.Fatal
- Change deleteTask to only remove database records, preserve downloaded files
- Add paused status for interrupted tasks on shutdown

Co-Authored-By: Claude

internal/router/task.go

@@ -1,11 +1,9 @@
 package router
 
 import (
-	"database/sql"
 	"encoding/json"
 	"fmt"
 	"net/http"
-	"os"
 	"os/exec"
 	"runtime"
 	"strconv"
@@ -177,30 +175,8 @@ func deleteTask(w http.ResponseWriter, r *http.Request) {
 	}{}
 
 	for _, taskID := range taskIDs {
-		_task, err := task.GetTask(db, taskID)
-		if err == sql.ErrNoRows {
-			// 数据库中没有该条记录，忽略
-			successCount++
-			continue
-		}
-		if err != nil {
-			failedTasks = append(failedTasks, struct {
-				ID    int
-				Error string
-			}{ID: taskID, Error: fmt.Sprintf("获取任务失败: %v", err)})
-			continue
-		}
-		filePath := _task.FilePath()
-		err = os.Remove(filePath)
-		if err != nil && !os.IsNotExist(err) {
-			failedTasks = append(failedTasks, struct {
-				ID    int
-				Error string
-			}{ID: taskID, Error: fmt.Sprintf("文件删除失败: %v", err)})
-			continue
-		}
-
-		err = task.DeleteTask(db, taskID)
+		// 只删除数据库记录，不删除已下载的视频文件
+		err := task.DeleteTask(db, taskID)
 		if err != nil {
 			failedTasks = append(failedTasks, struct {
 				ID    int