Strip dead Windows-only code paths, add LTO/march build flags, seed default GM account

Removes now-unreachable #ifdef _WIN32 branches across Common/ and the three
server binaries (all builds are Linux-only), deletes the entirely unused
SEH exception-handling shim and an orphaned vendored MySQL client header.
Enables LTO and -march=x86-64-v2 in Release builds for a small free perf win.

Also seeds a default GM account (gmadmin/changeme) via
database sql files/gm_account.sql so a fresh deploy has something to log
into the web admin panel with, switches the MariaDB data directory from a
Docker named volume to a host bind mount for visibility/backup, and expands
the README with a fuller Docker deployment walkthrough.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Src/Common/SocketClient.cpp

@@ -19,7 +19,6 @@
     developed with Main erose/hrose source server + some change from the original eich source
 */
 #include "sockets.h"
-#include "seh.hpp"
 #include "config.h"
 
 // Constructor
@@ -183,16 +182,6 @@ void CClientSocket::SendPacket( CPacket *P )
         Log(MSG_WARNING,"tried to send a packet to a socket that doesn't exist any more");
         return;
     }
-    //__seh_try
-    //{
-    //    EncryptBuffer( CryptTable, Buffer );
-    //}
-    //__seh_except(info, context)
-    //{
-    //    Log(MSG_WARNING,"Caught a bad packet %s. Send cancelled", Buffer);
-    //    return;
-    //}
-    //__seh_end
 	EncryptBuffer( CryptTable, Buffer2 );
 #endif
 	send( sock, (char*)Buffer2, Size, 0 );