fix(ipam): require resource name before allocating IP and fill missing fields on VM link

- Validate resource name in auto-allocate form before sending request
- Fill resource_name, proxmox_node, resource_type, mac_address when syncing
  a VM that already has matching vmid/server_id but missing metadata

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/ipam_service.py

@@ -707,8 +707,16 @@ class IPAMService:
                     last_seen=utcnow()
                 )
             else:
-                # Just update last_seen
+                # vmid/server already match — update last_seen and fill missing resource_name/node
                 existing.last_seen = utcnow()
+                if not existing.resource_name and vm_name:
+                    existing.resource_name = vm_name
+                if not existing.proxmox_node and node:
+                    existing.proxmox_node = node
+                if not existing.resource_type and vm_type:
+                    existing.resource_type = vm_type
+                if mac_address and not existing.mac_address:
+                    existing.mac_address = mac_address
                 self.db.commit()
                 return existing, None