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>

CMakeLists.txt

@@ -34,17 +34,28 @@ add_compile_options(
     # in Release builds only). -fwrapv makes overflow well-defined instead
     # of auditing every arithmetic expression in the tree.
     -fwrapv
+    # x86-64-v2 (SSE4.2/POPCNT baseline, ~2009+ CPUs) is safe for any real
+    # deployment target and gets a bit more codegen than the generic x86-64
+    # baseline GCC defaults to.
+    -march=x86-64-v2
     -include "${CMAKE_SOURCE_DIR}/Src/Common/compat.h"
 )
 add_compile_definitions(_CRT_SECURE_NO_WARNINGS)
 
+# Link-time optimization: lets GCC inline/optimize across the many small
+# .cpp translation units instead of only within each one.
+include(CheckIPOSupported)
+check_ipo_supported(RESULT IPO_SUPPORTED OUTPUT IPO_ERROR)
+if(IPO_SUPPORTED AND CMAKE_BUILD_TYPE STREQUAL "Release")
+    set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON)
+endif()
+
 set(COMMON_SOURCES
     "Src/Common/config.cpp"
     "Src/Common/database/database.cpp"
     "Src/Common/globals.cpp"
     "Src/Common/log.cpp"
     "Src/Common/rosecrypt.cpp"
-    "Src/Common/seh.cpp"
     "Src/Common/SocketBase.cpp"
     "Src/Common/SocketClient.cpp"
     "Src/Common/SocketServer.cpp"