Fix PXE interface detection, add br-pxe bridge to test VM, network upload import

- Playbook: detect interface already configured with 10.9.100.1 before
  falling back to non-default-gateway heuristic (fixes dnsmasq binding
  to wrong NIC when multiple interfaces exist)
- test-vm.sh: auto-attach br-pxe bridge NIC if available on host
- Webapp: add network upload import via SMB share with shared driver
  deduplication and symlinks

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

test-vm.sh

@@ -138,9 +138,15 @@ echo "    Extracted vmlinuz and initrd from casper/"
 echo ""
 echo "[4/4] Launching VM ($VM_NAME)..."
 
-# Use the default libvirt network (NAT, 192.168.122.0/24)
-# The VM gets a DHCP address initially for install access.
-# The Ansible playbook will later configure 10.9.100.1/24 for production use.
+# Use the default libvirt network (NAT, 192.168.122.0/24) for install access.
+# If br-pxe bridge exists, add a second NIC for the isolated PXE switch.
+# The Ansible playbook will configure 10.9.100.1/24 on the PXE interface.
+PXE_BRIDGE_ARGS=""
+if ip link show br-pxe &>/dev/null; then
+    PXE_BRIDGE_ARGS="--network bridge=br-pxe,model=virtio"
+    echo "    Found br-pxe bridge, adding isolated switch NIC"
+fi
+
 virt-install \
     --name "$VM_NAME" \
     --memory "$VM_RAM" \
@@ -149,6 +155,7 @@ virt-install \
     --disk path="$UBUNTU_ISO",device=cdrom,readonly=on \
     --disk path="$CIDATA_ISO",device=cdrom \
     --network network=default \
+    $PXE_BRIDGE_ARGS \
     --os-variant ubuntu24.04 \
     --graphics none \
     --console pty,target_type=serial \