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_event.go

@@ -142,7 +142,7 @@ func handleMsgMhfGetKeepLoginBoostStatus(s *Session, p mhfpacket.MHFPacket) {
 	}
 	for rows.Next() {
 		var temp loginBoost
-		rows.StructScan(&temp)
+		_ = rows.StructScan(&temp)
 		loginBoosts = append(loginBoosts, temp)
 	}
 	if len(loginBoosts) == 0 {