fix(Core): pass Battleground arg to CanJoinToBattleground

Player::CanJoinToBattleground now requires a Battleground const* argument.
Pass the resolved bg template to fix the build.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: Nyeriah

src/LowLevelRBG.cpp

@@ -132,7 +132,7 @@ class low_level_rbg_commandscript : public CommandScript
         {
             err = ERR_LFG_CANT_USE_BATTLEGROUND;
         }
-        else if (!player->CanJoinToBattleground()) // has deserter debuff
+        else if (!player->CanJoinToBattleground(bgt)) // has deserter debuff
         {
             err = ERR_GROUP_JOIN_BATTLEGROUND_DESERTERS;
         }