What is the ideal fridge temperature
I notice that in both countries where I lived the fridges are kept at 6 or 7 °C (43-45 °F). When it would be colder, people find their drinks to cold, when served straight out of fridge (especially in winter). Also a lower temperature would cost more energy.
What would be the best temperature for food storage. Would that be even lower? For storing meals, how much time longer would food stay good when the fridge would be colder?
The USDA says refrigeration temperature should be 40°F (4.4°C) or below. If food is in there at a higher temperature (such as the 43-45°F the question mentions), for longer than 2 hours, and they're saying the food isn't safe.
Keep in mind that when too cold, after a while parts of things freeze, which can damage items, or severely diminish their pleasant taste. This especially goes for produce such as lettuce or even tomatoes. If you want to store leftovers longer, consider freezing them.
Edit: corrected to 40°F or below
Actually that link says "40 F or below", which is an important difference. In practice, I'd recommend aiming for around 36 so that you have some headroom, and to allow for variation in different parts of the fridge. That also leaves you a few degrees above freezing so you don't inadvertently ruin anything that way either.