Best within minutes of finishing; it stiffens as it cools. Leftovers keep 2 days — reheat with extra broth and stir, or better, make arancini: roll cold risotto into balls, crumb, and fry.
Not constantly, but frequently. Stirring rubs the grains together and releases the starch that makes risotto creamy. Occasional stirring gives you rice in soup.
Tender with the faintest resistance at the centre, and it should spread slowly when you tilt the pan — Italians call it all'onda, 'wavy'. Stiff risotto is overcooked or under-liquided.
Restaurants par-cook it: stop at 15 minutes, spread on a tray to cool, then finish with the last ladles of broth to order. That works well at home too.