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>

.gitignore

@@ -2,6 +2,11 @@ build/
 build-*/
 .env
 
+# MariaDB data directory -- now a bind mount (was a Docker named volume) so
+# it's visible/backup-able from the host, but that also means it lives
+# inside the repo checkout and must never be committed.
+/docker/data/
+
 # Windows game client + data assets, kept locally for end-to-end login/game
 # testing but not part of the server source tree. Binary/3DData is the
 # separate, already-pruned subset of this data the server itself actually