Expose noble readiness as a public API value

[legoktm]

Description

The SI's public /metadata endpoint outputs some information about a SecureDrop install that is used by us for various ecosystem observation purposes (primarily checking that auto-updates have succeeded).

With the upcoming noble migration, we are looking at doing it automatically, but there will still be some issues that cannot be fixed ahead of time (e.g. enough free disk space).

My idea is that we add a flag to the API output like noble_readiness: true/false. It only outputs a boolean flag, which we can use to find instances that are not ready, and reach out to their admins, who can log in and run a script that outputs more details on what's wrong.

Example checks that would trigger a false response:

• enough free diskspace on the root partition to download new packages
• migrated users out of the ssh group (see #7317)
• removed ufw firewall (#7313)
• check no other unexpected apt sources

The main argument against this is that we are exposing information about an instances internal health, we mitigate that by just outputting a boolean, and checking enough things that a potential attacker cannot figure out which thing is triggering the failure.

Also because this is running from the app server, I don't think we can handle the mon server.

[zenmonkeykstop]

Overall I don't think this poses much of a security risk. The set of changes an attacker might be able to infer is smaller than it seems tho. 2/4 changes (ufw,ssh) above could be inferred by looking at version number, as they'd be enforced by a release's postint. Other ones might be useful to an attacker looking to, say, use up disk space - but they could just put in lots of submissions.

I'm kindof of two minds about this approach tho, just because it is a public API, this is extra non-public info (albeit collapsed into a boolean). One alternative - which is in line with what was done on previous updates - would be to consider a banner message in the JI. This can be a lot more verbose - and while we won't be able to see it, if it's sufficiently "alerty" it will enlist the help of users to get admins to contact us rather than the other way round.

The other advantage of an alert within the JI is that for folks running an instance without contact with FPF, the API change is unlikely to be noticed, while the JI banner is going be visible to anyone using the system. (Obviously, they are not mutually exclusive.)

[legoktm]

I'll start working on the banner message. I'm also thinking about an OSSEC alert since that can check the mon server as well.