Add Jellyfin account migration

pages/02.linux/jellyfin/default.en.md

@@ -114,3 +114,22 @@ Enable the config and restart nginx
 ln -s /etc/nginx/sites-available/(config) /etc/nginx/sites-enabled/
 systemctl restart nginx
 ```
+
+## Migrate Authentication
+
+There are two possible ways to migrate user accounts to a different authentication backend.  
+I have personally used the first option, but the second option might work as well
+
+### Option one: Match Jellyfin Username
+
+The first option is simply matching the Jellyfin username to the username, UID or other provided by the authentication backend.  
+After setting the username, simply switching the authentication provider allowed logging in with the new authentication backend.
+
+### Option two: Copy Progress from library.db
+
+For the second option, a new user account is created.  
+The progress of the old user account will be copied to this newly created account.
+
+A guide for the process can be found on Reddit.
+
+> [How to transfer users from one instance of Jellyfin to another?](https://old.reddit.com/r/jellyfin/comments/ejat65/how_to_transfer_users_from_one_instance_of/)

pages/04.other/useful-commands/default.en.md

@@ -18,6 +18,22 @@ convert -density 600 {INPUT.PDF} -crop 50x100% +repage {OUT.PDF}
 -crop 50x100%: this splits the pdf into "left" and "right". 100x50% would split into "top" and "bottom"
 ```
 
+## Ghostscript
+
+### Merge multiple PDF files
+
+Ghostscript is preinstalled on a lot of Linux systems and can quite easily be used to merge and optimize multiple PDF files into one.
+
+[shuser]
+
+```sh
+gs -q -sDEVICE=pdfwrite -dPDFSETTINGS=/prepress -o merged.pdf <first>.pdf <second>.pdf
+```
+
+[/shuser]
+
+> [Merge / convert multiple PDF files into one PDF](https://stackoverflow.com/a/19358402)
+
 ## Find
 
 ### Change filtered permissions