Add server control (start/stop/restart, live logs, broadcast, kick) to webadmin

Docker SDK drives login/char/worldserver containers via docker.sock (mounted
read-only into webadmin); live logs stream over SSE with nginx buffering
disabled so lines arrive in real time. Broadcast/kick go through a new
admin_queue DB table that the world server polls every ~2s in its existing
WorldProcess tick loop, avoiding a second IPC channel to the game server.

webadmin/requirements.txt

@@ -1,3 +1,4 @@
 Flask==3.0.3
 PyMySQL==1.1.1
 gunicorn==22.0.0
+docker==7.1.0