Reduce handler request boilerplate

- Add session.Attach to reuse per-request sessions across middleware and ghome handlers
- Make Session finalize/abort paths idempotent to avoid duplicate commit or rollback work
- Move request_data parsing helper to internal/utils
- Expand ParseRequestData usage across api, download, unit, live, multiunit, and subscenario handlers
- Extract download package metadata into internal/handler/download/types.go
- Return explicit errors from selected session lookup helpers and update callers
- Make api action handlers return errors directly instead of calling ss.CheckErr in nested layers

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/handler/api/museum/info.go

@@ -21,8 +21,8 @@ func museumInfo(ctx *gin.Context) (res any, err error) {
 	var smileBuff, pureBuff, coolBuff int
 	err = ss.MainEng.Table("museum_contents_m").Cols("museum_contents_id,smile_buff,pure_buff,cool_buff").
 		OrderBy("museum_contents_id ASC").Find(&museumRes)
-	if ss.CheckErr(err) {
-		return
+	if err != nil {
+		return nil, err
 	}
 
 	for _, res := range museumRes {