physics: add server flight-back speed override to CameraController

SMSG_FORCE_FLIGHT_BACK_SPEED_CHANGE was already ACK'd and stored in
serverFlightBackSpeed_, but the value was never accessible or synced
to the CameraController. Backward flight movement always used forward
flight speed (flightSpeedOverride_), making it faster than the server
intended.

- Add getServerFlightBackSpeed() accessor in GameHandler
- Add flightBackSpeedOverride_ field and setter in CameraController
- Apply it in the fly movement block: backward-only flight uses the
  back speed; forward or strafing uses the forward speed as WoW does
- Fallback: 50% of forward flight speed when override is unset
- Sync per-frame in application.cpp alongside the other speed overrides

src/core/application.cpp

@@ -1013,6 +1013,7 @@ void Application::update(float deltaTime) {
                     renderer->getCameraController()->setSwimSpeedOverride(gameHandler->getServerSwimSpeed());
                     renderer->getCameraController()->setSwimBackSpeedOverride(gameHandler->getServerSwimBackSpeed());
                     renderer->getCameraController()->setFlightSpeedOverride(gameHandler->getServerFlightSpeed());
+                    renderer->getCameraController()->setFlightBackSpeedOverride(gameHandler->getServerFlightBackSpeed());
                     renderer->getCameraController()->setRunBackSpeedOverride(gameHandler->getServerRunBackSpeed());
                     renderer->getCameraController()->setMovementRooted(gameHandler->isPlayerRooted());
                     renderer->getCameraController()->setGravityDisabled(gameHandler->isGravityDisabled());