Add rcon.py for running server commands (op, gamemode, etc)

No interactive console exists since lazymc/systemd run the server in
the background, and mcrcon isn't packaged for Debian 13. Plain
Python3 RCON client instead, reads the password straight from
server.properties. setup-server.sh now copies it into mcserver/.

server/rcon.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+"""Minimal Minecraft RCON client. Usage: rcon.py "<command>" ["<command2>" ...]"""
+import socket
+import struct
+import sys
+
+HOST = "127.0.0.1"
+PORT = 25575
+PASSWORD_FILE = "server.properties"
+
+
+def read_password():
+    with open(PASSWORD_FILE) as f:
+        for line in f:
+            if line.startswith("rcon.password="):
+                return line.strip().split("=", 1)[1]
+    raise SystemExit("rcon.password not found in server.properties")
+
+
+def send_packet(sock, req_id, pkt_type, payload):
+    data = struct.pack("<ii", req_id, pkt_type) + payload.encode() + b"\x00\x00"
+    sock.sendall(struct.pack("<i", len(data)) + data)
+
+
+def read_packet(sock):
+    length = struct.unpack("<i", sock.recv(4))[0]
+    data = b""
+    while len(data) < length:
+        data += sock.recv(length - len(data))
+    req_id, pkt_type = struct.unpack("<ii", data[:8])
+    payload = data[8:-2].decode(errors="replace")
+    return req_id, pkt_type, payload
+
+
+def main():
+    password = read_password()
+    sock = socket.create_connection((HOST, PORT), timeout=10)
+    send_packet(sock, 1, 3, password)  # SERVERDATA_AUTH
+    req_id, _, _ = read_packet(sock)
+    if req_id == -1:
+        raise SystemExit("RCON auth failed")
+
+    for i, cmd in enumerate(sys.argv[1:], start=2):
+        send_packet(sock, i, 2, cmd)  # SERVERDATA_EXECCOMMAND
+        _, _, response = read_packet(sock)
+        print(f"> {cmd}\n{response}")
+
+
+if __name__ == "__main__":
+    main()