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/live/schedule.go

@@ -14,8 +14,8 @@ func liveSchedule(ctx *gin.Context) (res any, err error) {
 	var liveID []int
 	err = ss.MainEng.Table("special_live_m").
 		Cols("live_difficulty_id").OrderBy("live_difficulty_id ASC").Find(&liveID)
-	if ss.CheckErr(err) {
-		return
+	if err != nil {
+		return nil, err
 	}
 
 	liveList := []liveapischema.LiveList{}

internal/handler/api/live/status.go

@@ -14,8 +14,8 @@ func liveStatus(ctx *gin.Context) (res any, err error) {
 	var liveDifficultyID []int
 	normalLives := []liveapischema.NormalLiveStatusList{}
 	err = ss.MainEng.Table("normal_live_m").Cols("live_difficulty_id").OrderBy("live_difficulty_id ASC").Find(&liveDifficultyID)
-	if ss.CheckErr(err) {
-		return
+	if err != nil {
+		return nil, err
 	}
 	for _, id := range liveDifficultyID {
 		normalLive := liveapischema.NormalLiveStatusList{
@@ -31,8 +31,8 @@ func liveStatus(ctx *gin.Context) (res any, err error) {
 
 	specialLives := []liveapischema.SpecialLiveStatusList{}
 	err = ss.MainEng.Table("special_live_m").Cols("live_difficulty_id").OrderBy("live_difficulty_id ASC").Find(&liveDifficultyID)
-	if ss.CheckErr(err) {
-		return
+	if err != nil {
+		return nil, err
 	}
 	for _, id := range liveDifficultyID {
 		specialLive := liveapischema.SpecialLiveStatusList{