Com: add db upgrade comment

internal/db/update.go

@@ -38,12 +38,14 @@ var dbVersions = map[string]dbVersion{
 	"0.0.2": {
 		NextVersion: "0.0.3",
 		Upgrade: func(db *gorm.DB) error {
+			// delete all vendors, because we are going to change the more vendor table, e.g. bilibili_vendors
 			return db.Migrator().DropTable("streaming_vendor_infos")
 		},
 	},
 	"0.0.3": {
 		NextVersion: "",
 		Upgrade: func(db *gorm.DB) error {
+			// alist and emby movies path are changed, so we need to delete them
 			_ = db.Exec("DELETE FROM movies WHERE base_vendor_info_vendor IN ('alist', 'emby')").Error
 			return db.Migrator().DropTable("alist_vendors", "emby_vendors")
 		},