Teach the PocketBase bot every field

The bot covered most of script_scripts but not categories, has_arm,
execute_in or app_vars, so those had to be edited by hand in the admin UI.

categories is a relation, so names are resolved against script_categories
and ids are accepted too - a copy out of the PocketBase UI works either way.
app_vars is a JSON column and rides the code-block 'set' path with its
content parsed rather than stored verbatim, so readers get an object.

cpu/ram/hdd/os/version live inside install_methods, and '/pocketbase immich
hdd=25' is how people ask for them. They now route to the default (non-Alpine)
method instead of being rejected as unknown fields, the reply names which
method was touched, and the value syncs into ct/<slug>.sh like the 'method'
path already does.

Author: MickLesk