Add screenshots and type to the bot

Two gaps were left. type is a relation to z_ref_script_types, so it needs the
same name-to-id resolution as categories - people write "ct", not a
fifteen-character id. Screenshots had no command at all.

The screenshot subcommand hands the URLs to the frontend's /api/screenshots
rather than fetching images inside a workflow. That endpoint already checks
the content type and size and attaches the file to PocketBase; doing it a
second time here would be a second set of bugs. It needs
SCREENSHOT_IMPORT_SECRET, and says so plainly when it is missing instead of
failing halfway.

slug stays deliberately out of reach. It is the URL, the JSON filename and the
ct/<slug>.sh path at once, so renaming it is a migration rather than an edit,
and the help text now says that instead of leaving people to wonder.

Author: MickLesk