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/util/db.go

@@ -4,7 +4,6 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
-	"log"
 	"strings"
 	"sync"
 )
@@ -115,18 +114,28 @@ func SaveDownloadFolder(db *sql.DB, downloadFolder string) error {
 
 var SqliteLock sync.Mutex
 
-func MustGetDB(path ...string) *sql.DB {
-	pathStr := ""
-	if len(path) == 0 {
-		pathStr = "./data.db"
-	} else if len(path) > 1 {
-		log.Fatalln(errors.New("len(path) <= 1"))
-	} else {
+// GetDB 获取数据库连接，返回错误而非直接退出
+func GetDB(path ...string) (*sql.DB, error) {
+	pathStr := "./data.db"
+	if len(path) > 0 {
+		if len(path) > 1 {
+			return nil, errors.New("len(path) must be <= 1")
+		}
 		pathStr = path[0]
 	}
 	db, err := sql.Open("sqlite", pathStr)
 	if err != nil {
-		log.Fatalln("sql.Open:", err)
+		return nil, fmt.Errorf("sql.Open: %w", err)
+	}
+	return db, nil
+}
+
+// MustGetDB 获取数据库连接，失败时 panic（用于初始化阶段）
+// 推荐在 main 函数中使用 GetDB 并处理错误
+func MustGetDB(path ...string) *sql.DB {
+	db, err := GetDB(path...)
+	if err != nil {
+		panic(fmt.Sprintf("MustGetDB: %v", err))
 	}
 	return db
 }