[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

backend/app/services/virtual_printer/ssdp_server.py

@@ -232,7 +232,13 @@ class VirtualPrinterSSDPServer:
         try:
             msg = self._build_notify_message()
             self._socket.sendto(msg, (SSDP_BROADCAST_ADDR, SSDP_PORT))
-            logger.debug("Sent SSDP NOTIFY for %s", self.name)
+            logger.debug(
+                "Sent SSDP NOTIFY for %s (Location=%s, USN=%s, bind=%s)",
+                self.name,
+                self._get_local_ip(),
+                self.serial,
+                self._bind_ip,
+            )
         except OSError as e:
             logger.debug("Failed to send NOTIFY for %s: %s", self.name, e)
 
@@ -278,7 +284,13 @@ class VirtualPrinterSSDPServer:
             try:
                 response = self._build_response_message()
                 self._socket.sendto(response, addr)
-                logger.info("Sent SSDP response to %s for virtual printer '%s'", addr[0], self.name)
+                logger.info(
+                    "Sent SSDP response to %s for '%s' (Location=%s, USN=%s)",
+                    addr[0],
+                    self.name,
+                    self._get_local_ip(),
+                    self.serial,
+                )
             except OSError as e:
                 logger.debug("Failed to send SSDP response for %s: %s", self.name, e)