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>

database sql files/gm_account.sql

@@ -0,0 +1,9 @@
+-- Default GM account so a fresh deploy has something to log into the
+-- web admin panel and the game with. Change the password immediately on
+-- any server that isn't purely local/dev -- see README.md.
+--
+-- username: gmadmin
+-- password: changeme  (stored as md5, matching Src/Login Server/loginpackets.cpp
+--                      and webadmin/app.py's hash_password())
+INSERT INTO `accounts` (`username`, `password`, `accesslevel`, `lastlogin`, `logtime`)
+VALUES ('gmadmin', MD5('changeme'), 900, '', 0);