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/session/login.go

@@ -2,15 +2,15 @@ package session
 
 import loginmodel "honoka-chan/internal/model/login"
 
-func (ss *Session) GetAuthKey(token string) (bool, *loginmodel.AuthKey) {
+func (ss *Session) GetAuthKey(token string) (bool, *loginmodel.AuthKey, error) {
 	authKeyData := loginmodel.AuthKey{}
 	has, err := ss.UserEng.Table(new(loginmodel.AuthKey)).
 		Where("authorize_token = ?", token).Get(&authKeyData)
-	if ss.CheckErr(err) {
-		return false, nil
+	if err != nil {
+		return false, nil, err
 	}
 
-	return has, &authKeyData
+	return has, &authKeyData, nil
 }
 
 func (ss *Session) SetAuthKey(authKey *loginmodel.AuthKey) {