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

include/game/game_handler.hpp

@@ -1156,6 +1156,7 @@ public:
     float getServerSwimSpeed() const { return serverSwimSpeed_; }
     float getServerSwimBackSpeed() const { return serverSwimBackSpeed_; }
     float getServerFlightSpeed() const { return serverFlightSpeed_; }
+    float getServerFlightBackSpeed() const { return serverFlightBackSpeed_; }
     float getServerRunBackSpeed() const { return serverRunBackSpeed_; }
     bool isPlayerRooted() const {
         return (movementInfo.flags & static_cast<uint32_t>(MovementFlags::ROOT)) != 0;