Is it safe to constantly charge a LIR2032 ?

No. See batteryuniversity.com for a detailed discussion, page BU-409.
By the way, the charging circuit on those RTCs is an extremely bad design that can cause the DS3231 to malfunction. A number of posts on this forum and elsewhere describe some simple mods to remove all the junk, so that you can use a standard coin cell as intended. It will last for years.