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/unit/all.go

@@ -15,8 +15,8 @@ func unitAll(ctx *gin.Context) (res any, err error) {
 	unitMap := []unitmodel.UnitDataMap{}
 	err = ss.GetBasicUnitInfo().
 		Where("a.user_id = ?", ss.UserID).Find(&unitMap)
-	if ss.CheckErr(err) {
-		return
+	if err != nil {
+		return nil, err
 	}
 
 	unitsData := []unitapischema.Active{}