feat: mark events as favorites

[maakinoh]

Closes: #276.

This PR introduces new endpoints and a database table for marking events as favorites.

The nice to have feature about 'sharing favorite events' is probably best handled in a separate PR.

[Metawolve]

This looks like a reasonable solution.
But maybe a suggestion - not sure which way is actually better, both would be fine for me - but another way would be to have a direct n:m relationship between users and events. Meaning that the EventRecord would have a new property that is a list of users (e.g. "FavoredBy") and the UserRecord will have a new property that is a list of events (e.g. "FavoriteEvents"). The names are just suggestions, there's probably a better way to name them.
But this would mean that you wouldn't need an EventFavoriteRecord class, as EF Core will handle this automatically and create a relation table in the database (probably called UserRecordsEventRecords or something, although you can customize the name in the modelbuilder).
You then can just add events to favorites by just adding them to the list of FavoriteEvents of the user, and remove them by just removing them from that list.
Then you also could easily get the list of users that marked an event as favorite, by just getting event.FavoredBy, e.g. to show who favored an event in the app, if we want to do that in the future.

[maakinoh]

Good point, your approach would be more clean.

I wasn't quite sure if the UserRecord was still in active use, apart from the TG bot, and actively filled with data.
But I think it shouldn't be an issue to add the users' IDP Id to this table then, right? Dunno if this approach would mess somewhere.

[Metawolve]

Looks good apart from some minor comments

[Metawolve]

Lgtm