feat(proxmox): add journalctl streaming API endpoint for LXC containers

Add new /api/v1/proxmox/journalctl/:node/:vmid/:service endpoint that
streams real-time journalctl output from Proxmox LXC containers via
WebSocket connection. This enables live monitoring of container services
from the GoDoxy WebUI.

Implementation includes:
- New proxmox API handler with path parameter validation
- WebSocket upgrade for streaming output
- LXCCommand helper for executing commands over Proxmox VNC websocket
- LXCJournalctl wrapper for convenient journalctl -u service -f invocation
- Updated API documentation with proxmox integration

internal/proxmox/lxc_command.go

@@ -0,0 +1,110 @@
+package proxmox
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io"
+
+	"github.com/gorilla/websocket"
+)
+
+var ErrNoSession = fmt.Errorf("no session found, make sure username and password are set")
+
+// LXCCommand connects to the Proxmox VNC websocket and streams command output.
+// It returns an io.ReadCloser that streams the command output.
+func (n *Node) LXCCommand(ctx context.Context, vmid int, command string) (io.ReadCloser, error) {
+	if !n.client.HasSession() {
+		return nil, ErrNoSession
+	}
+
+	node, err := n.client.Node(ctx, n.name)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get node: %w", err)
+	}
+
+	term, err := node.TermProxy(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get term proxy: %w", err)
+	}
+
+	send, recv, errs, close, err := node.TermWebSocket(term)
+	if err != nil {
+		return nil, fmt.Errorf("failed to connect to term websocket: %w", err)
+	}
+
+	handleSend := func(data []byte) error {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case send <- data:
+			return nil
+		case err := <-errs:
+			return fmt.Errorf("failed to send: %w", err)
+		}
+	}
+
+	// Send command: `pct exec <vmid> -- <command>`
+	cmd := fmt.Appendf(nil, "pct exec %d -- %s\n", vmid, command)
+	if err := handleSend(cmd); err != nil {
+		return nil, err
+	}
+
+	// Create a pipe to stream the websocket messages
+	pr, pw := io.Pipe()
+
+	shouldSkip := true
+
+	// Start a goroutine to read from websocket and write to pipe
+	go func() {
+		defer close()
+		defer pw.Close()
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case msg := <-recv:
+				// skip the header message like
+
+				// Linux pve 6.17.4-1-pve #1 SMP PREEMPT_DYNAMIC PMX 6.17.4-1 (2025-12-03T15:42Z) x86_64
+				//
+				// The programs included with the Debian GNU/Linux system are free software;
+				// the exact distribution terms for each program are described in the
+				// individual files in /usr/share/doc/*/copyright.
+				//
+				// Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
+				// permitted by applicable law.
+				//
+				// root@pve:~# pct exec 101 -- journalctl -u "sftpgo" -f
+				//
+				// send begins after the line above
+				if shouldSkip {
+					if bytes.Contains(msg, cmd[:len(cmd)-2]) { // without the \n
+						shouldSkip = false
+					}
+					continue
+				}
+				if _, err := pw.Write(msg); err != nil {
+					return
+				}
+			case err := <-errs:
+				if err != nil {
+					if websocket.IsUnexpectedCloseError(err, websocket.CloseGoingAway, websocket.CloseAbnormalClosure) {
+						_ = pw.Close()
+						return
+					}
+					_ = pw.CloseWithError(err)
+					return
+				}
+			}
+		}
+	}()
+
+	return pr, nil
+}
+
+// LXCJournalctl streams journalctl output for the given service.
+func (n *Node) LXCJournalctl(ctx context.Context, vmid int, service string) (io.ReadCloser, error) {
+	return n.LXCCommand(ctx, vmid, fmt.Sprintf("journalctl -u %q -f", service))
+}