fix(channelserver): return error from Save() to prevent misleading success logs

CharacterSaveData.Save() silently returned on failure (nil decompressed
data, compression error, DB error) while the caller unconditionally
logged "Saved character data successfully". This made diagnosing save
failures difficult (ref #163).

Save() now returns an error, and all six call sites check it. The
success log in saveAllCharacterData only fires when the save actually
persisted.

server/channelserver/handlers_tower.go

@@ -376,7 +376,9 @@ func handleMsgMhfPostTenrouirai(s *Session, p mhfpacket.MHFPacket) {
 		sd, err := GetCharacterSaveData(s, s.charID)
 		if err == nil && sd != nil {
 			sd.RP -= pkt.DonatedRP
-			sd.Save(s)
+			if err := sd.Save(s); err != nil {
+				s.logger.Error("Failed to save RP after tower donation", zap.Error(err))
+			}
 			result, err := s.server.towerService.DonateGuildTowerRP(pkt.GuildID, pkt.DonatedRP)
 			if err != nil {
 				s.logger.Error("Failed to process tower RP donation", zap.Error(err))