fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

server/channelserver/handlers_rengoku.go

@@ -158,7 +158,7 @@ func handleMsgMhfEnumerateRengokuRanking(s *Session, p mhfpacket.MHFPacket) {
 	defer rows.Close()
 
 	for rows.Next() {
-		rows.StructScan(&score)
+		_ = rows.StructScan(&score)
 		if score.Name == s.Name {
 			bf.WriteUint32(i)
 			bf.WriteUint32(score.Score)