fix: reduce TimeoutStopSec from 660s to 15s

The backend shuts down in <1s. The 660s timeout was left from when
Bitcoin Core was managed by this service. With 660s, systemctl stop
hangs for 11 minutes if the process is already dead but systemd
hasn't noticed, blocking all deploys and restarts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

image-recipe/configs/archipelago.service

@@ -16,8 +16,8 @@ Restart=on-failure
 RestartSec=5
 WatchdogSec=300
 TimeoutStartSec=300
-# Bitcoin Core needs up to 600s to flush UTXO set on shutdown
-TimeoutStopSec=660
+# Backend shuts down in <1s; 15s is generous for any cleanup
+TimeoutStopSec=15
 
 # Filesystem protection
 ProtectSystem=strict