make (playlist_id, line_num) unique

store/types.go

@@ -157,8 +157,8 @@ type Files map[uint64]File
 
 type PlaylistEntry struct {
 	ID         uint   `json:"id" gorm:"primary_key"`
-	PlaylistID uint   `json:"-" gorm:"not null;index"`
-	Index      uint   `json:"index"`
+	PlaylistID uint   `json:"-" gorm:"not null;index;unique_index:unique_playlist_line_numbers"`
+	LineNum    uint   `json:"line-num" gorm:"not null;unique_index:unique_playlist_line_numbers"`
 	Uri        string `json:"uri"`
 	File       File   `json:"file"`
 	FileID     uint   `json:"-" gorm:"index"`