Fix virtual printer config changes ignored on running instances

sync_from_db() skipped VPs already in self._instances without checking
  if their config had changed. Mode, model, access code, bind IP, remote
  interface IP, and target printer changes were silently ignored until
  manual toggle off/on or full restart. Now detects config drift and
  restarts affected instances.

backend/app/services/virtual_printer/manager.py

@@ -626,6 +626,30 @@ class VirtualPrinterManager:
                         if printer:
                             proxy_ips[pvp.id] = (printer.ip_address, printer.serial_number)
 
+        # Detect config changes on running instances and restart if needed
+        for vp in enabled_vps:
+            instance = self._instances.get(vp.id)
+            if not instance:
+                continue
+
+            changed = (
+                instance.mode != vp.mode
+                or instance.model != (vp.model or DEFAULT_VIRTUAL_PRINTER_MODEL)
+                or instance.access_code != (vp.access_code or "")
+                or instance.bind_ip != (vp.bind_ip or "")
+                or instance.remote_interface_ip != (vp.remote_interface_ip or "")
+                or instance.target_printer_id != vp.target_printer_id
+            )
+
+            if changed:
+                logger.info(
+                    "VP %s config changed (mode: %s→%s), restarting",
+                    instance.name,
+                    instance.mode,
+                    vp.mode,
+                )
+                await self.remove_instance(vp.id)
+
         # Start instances for all enabled VPs (skip already running)
         for vp in enabled_vps:
             if vp.id in self._instances: