Stable v2.6.54

• fix(multiserver): server delete cleans up its IPs instead of a raw FK error
• fix(plesk-import): relocate misplaced subdomain files into the parent webspace
• fix(install): fresh-install audit — panel vhost idempotency, node health-check retry, imapsync pin
• docs(plan): mail deliverability suite — compromised-account detection shipped, MTA-STS/DMARC/TLS-RPT scoped
• feat(mail): compromised-account detection + auto-suspend (deliverability suite pt.1)