feat(proxmox): support node-level routes and journalctl access

This change enables Proxmox node-level operations without requiring a specific
LXC container VMID.

**Features added:**
- New `/proxmox/journalctl/{node}` API endpoint for streaming node journalctl
- Route configuration support for Proxmox nodes (VMID = 0)
- `ReverseLookupNode` function for node discovery by hostname/IP/alias
- `NodeJournalctl` method for executing journalctl on nodes

**Behavior changes:**
- VMID parameter in journalctl endpoints is now optional
- Routes targeting nodes (without specific containers) are now valid

**Bug fixes:**
- Fixed error message variable reference in route validation

internal/proxmox/node.go

@@ -186,3 +186,14 @@ func (n *Node) NodeCommand(ctx context.Context, command string) (io.ReadCloser,
 
 	return pr, nil
 }
+
+func (n *Node) NodeJournalctl(ctx context.Context, service string, limit int) (io.ReadCloser, error) {
+	command := "journalctl -f"
+	if service != "" {
+		command = fmt.Sprintf("journalctl -u %q -f", service)
+	}
+	if limit > 0 {
+		command = fmt.Sprintf("%s -n %d", command, limit)
+	}
+	return n.NodeCommand(ctx, command)
+}