Refactor: Update SQLAlchemy syntax to 2.0 to resolve LegacyAPIWarnings

[nesmomik]

When testing the RSVP endpoints with pytest a LegacyAPIWarning would show up in the test output.

• replaced Event.query.get_or_404(event_id) with db.get_or_404(Event, event_id)

• updated RSVP.query.filter_by(...) to use the explicit db.session.execute(db.select(RSVP)...) pattern