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/rendering/camera_controller.hpp

@@ -96,6 +96,7 @@ public:
     void setSwimSpeedOverride(float speed) { swimSpeedOverride_ = speed; }
     void setSwimBackSpeedOverride(float speed) { swimBackSpeedOverride_ = speed; }
     void setFlightSpeedOverride(float speed) { flightSpeedOverride_ = speed; }
+    void setFlightBackSpeedOverride(float speed) { flightBackSpeedOverride_ = speed; }
     void setRunBackSpeedOverride(float speed) { runBackSpeedOverride_ = speed; }
     void setMovementRooted(bool rooted) { movementRooted_ = rooted; }
     bool isMovementRooted() const { return movementRooted_; }
@@ -284,6 +285,7 @@ private:
     float swimSpeedOverride_ = 0.0f;
     float swimBackSpeedOverride_ = 0.0f;
     float flightSpeedOverride_ = 0.0f;
+    float flightBackSpeedOverride_ = 0.0f;
     float runBackSpeedOverride_ = 0.0f;
     // Server-driven root state: when true, block all horizontal movement input.
     bool movementRooted_ = false;