fix: check all Close() return values for errcheck lint

Add explicit error discards (_ =) for Close() calls on network
connections, SQL rows, and file handles across 28 files. Also add
.golangci.yml with standard linter defaults to match CI configuration.

server/channelserver/handlers_character.go

@@ -129,7 +129,7 @@ func GetCharacterSaveData(s *Session, charID uint32) (*CharacterSaveData, error)
 		s.logger.Error("Failed to get savedata", zap.Error(err), zap.Uint32("charID", charID))
 		return nil, err
 	}
-	defer result.Close()
+	defer func() { _ = result.Close() }()
 	if !result.Next() {
 		err = errors.New("no savedata found")
 		s.logger.Error("No savedata found", zap.Uint32("charID", charID))