Notify teams when hints answered

[qiaochloe]

I think the best way to do this is with a toast and server-sent events (SSEs). SSEs are a bit complicated but probably cleaner than email notifications or Discord pings. This should definitely be on the roadmap along with automatic guess updates.

[liteless]

Also worth noting that if the team is not online when the hint is responded to, we would need to notify them the next time they come online. We'd likely need to save something in the database to trigger the toast on next login.