Nice question...
Second part: you mean no net accn due to gravity, stable level flight. So you have the mass of the aircraft in equilibrium with the lift of the wings. So LIFT L is balanced by mass M and gravity g.
so L=Mg
Where does the energy to enable the above to work. For most powered flight from chemical release of combustion. For a jet engine this goes SUCK SQUEEZE BURN BLOW
Thrust.
Other methods: jump of a cliff in a hang glider: gravitation potential..you worked to get up on the cliff LOL. Your legs. A tow rope. A pull from another aircraft. Rocket propulsion as in Fat Albert.
Main thing about energy is that it transfers. Never lost only traded from one sort to another.
Need to consider stress and strain on the airframe too, a bit like the fulcrum in your question.