feat: add submission notification

[cubercsl]

Related to #2252, just for a POC now.

Lack of testing, and I'm not sure how much impact such a query would have on performance when there are a large number of teams polling for updates.

[meisterT]

is there a migration missing? seen doesn't exist yet, does it?

[cubercsl]

domjudge/webapp/src/Entity/Judging.php

Lines 118 to 120 in 4398ae5

	#[ORM\Column(options: ['comment' => 'Whether the team has seen this judging', 'default' => 0])]
	#[Serializer\Exclude]
	private bool $seen = false;

But this may not be important, because the logic of whether to send notifications does not look at this field, (which is at sendNotification @ domjudge.js)

[cubercsl]

BTW, this SQL query is the same as the one a user access /api/contests/:cid/judgements, except it filters out entries they have ever seen (it will be marked as seen when they click the submission).

Therefore, it is also feasible to implement it directly on the frontend js instead of adding a function to the /update endpoint.