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/background/info.go

@@ -13,14 +13,14 @@ func backgroundInfo(ctx *gin.Context) (res any, err error) {
 
 	var backgroundList []int
 	err = ss.MainEng.Table("background_m").Cols("background_id").Find(&backgroundList)
-	if ss.CheckErr(err) {
-		return
+	if err != nil {
+		return nil, err
 	}
 
 	var backgroundID int
 	_, err = ss.UserEng.Table("user_pref").Where("user_id = ?", ss.UserID).Cols("background_id").Get(&backgroundID)
-	if ss.CheckErr(err) {
-		return
+	if err != nil {
+		return nil, err
 	}
 
 	backgroundsList := []backgroundapischema.Info{}