Commit challs.txt once a day at 04:00, separate from the HedgeDoc refresh

The 30-minute timer now only updates the HedgeDoc note; a new
ctf-challs.timer runs --challs daily at 04:00. Locks are per target so
the two runs can overlap without one skipping.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ctf-challs.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Commit the testable challenges to challs.txt in the deployment repo
+Documentation=file://__DIR__/README.md
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+WorkingDirectory=__DIR__
+ExecStart=__DIR__/ctf_status.py --challs --quiet
+TimeoutStartSec=5min