Add Discord guild ID configuration and enhance event announcement options

This commit introduces the `DISCORD_GUILD_ID` environment variable to the configuration files, allowing for better integration with Discord for role synchronization. The event announcement functionality has been updated to include an option for the `@everyone` mention, which can be toggled during event creation. The backend logic has been modified to handle this new option, and corresponding updates have been made to the frontend to allow users to control the mention behavior. Additionally, tests have been added to ensure the correct functionality of these features.

backend/internal/adapter/postgres/store.go

@@ -312,6 +312,12 @@ func (s *Store) DeleteEvent(ctx context.Context, eventID string) error {
 	if _, err = tx.Exec(ctx, `DELETE FROM series WHERE body->>'eventId'=$1`, eventID); err != nil {
 		return err
 	}
+	if _, err = tx.Exec(ctx, `INSERT INTO discord_role_sync_jobs(event_id,action,generation,role_snapshot)
+		SELECT $1,'teardown',$2,COALESCE(jsonb_agg(discord_role_id) FILTER (WHERE discord_role_id IS NOT NULL),'[]'::jsonb)
+		FROM discord_managed_roles WHERE scope='event' AND event_id=$1`,
+		eventID, time.Now().UTC().UnixNano()); err != nil {
+		return err
+	}
 	tag, err := tx.Exec(ctx, `DELETE FROM events WHERE id=$1`, eventID)
 	if err != nil {
 		return err