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>

docker-compose.yml

@@ -42,7 +42,7 @@ services:
       MYSQL_USER: ${DB_USER:-rose}
       MYSQL_PASSWORD: ${DB_PASSWORD:-change_me}
     volumes:
-      - db-data:/var/lib/mysql
+      - "./docker/data/mysql:/var/lib/mysql"
       - "./database sql files:/docker-entrypoint-initdb.d:ro"
     healthcheck:
       test: ["CMD", "healthcheck.sh", "--connect", "--innodb_initialized"]
@@ -118,6 +118,5 @@ services:
         condition: service_healthy
 
 volumes:
-  db-data:
   logs:
   clanmark: