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_rengoku.go

@@ -159,7 +159,7 @@ func handleMsgMhfEnumerateRengokuRanking(s *Session, p mhfpacket.MHFPacket) {
 		doAckBufSucceed(s, pkt.AckHandle, make([]byte, 11))
 		return
 	}
-	defer rows.Close()
+	defer func() { _ = rows.Close() }()
 
 	for rows.Next() {
 		_ = rows.StructScan(&score)