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_festa.go

@@ -177,7 +177,7 @@ func handleMsgMhfInfoFesta(s *Session, p mhfpacket.MHFPacket) {
 	} else {
 		defer rows.Close()
 		for rows.Next() {
-			rows.Scan(&id, &start)
+			_ = rows.Scan(&id, &start)
 		}
 	}