PHPstan for update.freshrss.org

[math-GH]

How is the current level of the files for update.freshrss.org?

I think it could be useful to have the code here also on a high level.

( Ping @Alkarex and @ColonelMoutarde )

[Alkarex]

Probably, but the whole update mechanism should be reworked (e.g. to avoid manual processes, better work with branches, etc.)
I started a PR a long time ago FreshRSS/FreshRSS#1760 but did not have the motivation