[Fix] Virtual Printer FTP routed to wrong VP with different access codes (#735)

When running multiple virtual printers with different access codes on
  separate bind IPs, FTP connections were always routed to the wrong VP.

  Root cause: the iptables REDIRECT rule (990→9990) rewrites the
  destination IP to the incoming interface's primary address. With Linux's
  weak host model (arp_filter=0), packets for secondary IPs arrive on the
  primary interface, and REDIRECT sends them all to the first VP's FTP
  server. MQTT was unaffected because port 8883 had no redirect.

  Fix: FTP server now binds directly to port 990 (standard implicit FTPS),
  eliminating the iptables redirect entirely. Requires CAP_NET_BIND_SERVICE
  (already set in the systemd service file and Docker image).

  Also removed a global asyncio set_exception_handler() in the MQTT server
  that was overwritten by each VP instance, causing spurious "Unhandled
  exception in client_connected_cb" errors on startup.

  Changes:
  - FTP_PORT: 9990 → 990 (ftp_server.py)
  - Removed set_exception_handler() from MQTT server
  - Updated Dockerfile, docker-compose.yml port mappings
  - Deprecated --redirect-990 in install script
  - Updated wiki: removed iptables instructions for all platforms
  - Added migration guide (docs/migration-vp-ftp-port.md)
  - Added unit tests for port constant and no-global-state invariant

Dockerfile

@@ -57,7 +57,7 @@ EXPOSE 3000
 EXPOSE 3002
 EXPOSE 8000
 EXPOSE 8883
-EXPOSE 9990
+EXPOSE 990
 EXPOSE 50000-50100
 
 # Health check (uses PORT env var via shell)