ops: docker prune timer, kanboard backup/healthcheck, backup script fixes

docker-maintenance/ adds a systemd timer + prune.sh for the root LV that holds
Docker's data-root and has filled to 100% before, risking ENOSPC corruption.
The script sticks to the safe reclaim set (builder cache, dangling images,
stopped containers) and deliberately avoids `-a` and volume pruning, which can
destroy live data when run unattended.

kanboard/backup.sh and healthcheck.sh bring Kanboard in line with the other
services. seafile/ and vaultwarden/ backup scripts get fixes carried from the
stability audit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

seafile/backup.sh

@@ -33,11 +33,13 @@ ls "$DEST/"
 # Notify Zabbix
 if [[ -f /root/.zabbix_token ]]; then
   ZABBIX_TOKEN=$(cat /root/.zabbix_token)
-  curl -s -X POST http://localhost:81/api_jsonrpc.php \
+  NOW_EPOCH=$(date '+%s')
+  env -u HTTPS_PROXY -u HTTP_PROXY -u ALL_PROXY -u https_proxy -u http_proxy -u all_proxy \
+    curl -s -X POST http://192.168.1.4:81/api_jsonrpc.php \
     -H "Content-Type: application/json" \
     -H "Authorization: Bearer $ZABBIX_TOKEN" \
-    -d "{\"jsonrpc\":\"2.0\",\"method\":\"history.push\",\"id\":1,\"params\":{\"itemid\":\"70369\",\"value\":\"$(date '+%Y-%m-%d %H:%M')\"}}" > /dev/null \
-    && echo "Zabbix notified."
+    -d "{\"jsonrpc\":\"2.0\",\"method\":\"history.push\",\"id\":1,\"params\":{\"itemid\":\"70369\",\"value\":$NOW_EPOCH}}" > /dev/null \
+    && echo "Zabbix notified (seafile.backup.ts=$NOW_EPOCH)."
 fi
 
 # Rotate: keep last 5 backups