fix(security): authenticate VNC/terminal WebSockets and harden SSH/config

Critical security and robustness fixes from a code audit:

- WS terminal/VNC now require a JWT (?token=), validate the session,
  vms.console permission and VM ownership before bridging to Proxmox
  (previously the LXC terminal gave unauthenticated root shell access).
  Frontend ConsolePage passes the token in both WS URLs.
- Fix SSH fallback for `pvecm delnode`: SSHClient now supports password
  auth (it previously crashed with TypeError); ProxmoxClient stores the
  password for SSH fallbacks and the host port suffix is stripped.
- Add production startup warnings for default ADMIN_PASSWORD and wildcard
  CORS_ORIGINS.
- dashboard: use tz-aware utcnow() instead of naive datetime.utcnow().
- crypto: narrow decrypt_value except to InvalidToken and log failures.
- ssh_client: guard the shared connection cache with a lock; make
  unknown-host-key rejection opt-in via SSH_REJECT_UNKNOWN_HOSTS.
- logging: send loguru console output to stderr instead of stdout.
- Sync version fallback to 1.5.1; remove one-off refactor scripts.
- Add CODE_REVIEW.md audit report.

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

backend/.env.example

@@ -24,6 +24,9 @@ ACCESS_TOKEN_EXPIRE_MINUTES=1440
 SSH_TIMEOUT=10
 DEFAULT_SSH_USER=root
 DEFAULT_SSH_PORT=22
+# Reject SSH connections to hosts with unknown/changed host keys (mitigates MITM).
+# Enable in production once your servers' host keys are in known_hosts.
+SSH_REJECT_UNKNOWN_HOSTS=false
 
 # Security
 # In production set to your actual domain(s), e.g.: https://panel.example.com