physics: add server swim-back speed override to CameraController

Backward swimming was using 50% of forward swim speed as a hardcoded
fallback. Wire up the server-authoritative swim back speed so Warlock
Dark Pact, buffs, and server-forced speed changes all apply correctly
when swimming backward.

- game_handler.hpp: add getServerSwimBackSpeed() accessor
- camera_controller.hpp: add swimBackSpeedOverride_ field + setter
- camera_controller.cpp: apply swimBackSpeedOverride_ when player
  swims backward without forward input; fall back to 50% of swim speed
- application.cpp: sync swim back speed each frame

include/game/game_handler.hpp

@@ -1154,6 +1154,7 @@ public:
     float getServerRunSpeed() const { return serverRunSpeed_; }
     float getServerWalkSpeed() const { return serverWalkSpeed_; }
     float getServerSwimSpeed() const { return serverSwimSpeed_; }
+    float getServerSwimBackSpeed() const { return serverSwimBackSpeed_; }
     float getServerFlightSpeed() const { return serverFlightSpeed_; }
     float getServerRunBackSpeed() const { return serverRunBackSpeed_; }
     bool isPlayerRooted() const {