Upgrade pip in the update script before installing requirements

Matches what install-linkvault.sh already does on first setup.

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

scripts/proxmox/update-linkvault.sh

@@ -39,6 +39,7 @@ else
   git pull --ff-only
 fi
 
+.venv/bin/pip install -q --upgrade pip
 .venv/bin/pip install -q -r requirements.txt
 systemctl restart linkvault
 "