feat: interactive terminal — WS-bridged docker exec PTY (#46)

GET /services/:id/terminal upgrades to a WebSocket attached to a
`docker exec` PTY in the service's running replica. JSON frames both
ways: stdin/resize out, stdout/exit/error in. Shell probe tries each
candidate (bash→sh) since `command -v` needs a shell to run; distroless
images get a clean "no shell" error instead of an OCI failure dump.

- docker.Client gains ExecAttach + ExecResize wrappers
- Frontend: xterm.js replaces the one-off console section — live PTY,
  fit-addon resizes forwarded over the socket
- CLI: `containr shell <id>` bridges the local TTY (raw mode, SIGWINCH
  resize forwarding); `containr exec` keeps the one-off command path
- Access mirrors service logs: project manage scope required

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/cli/commands/exec.go

@@ -0,0 +1,113 @@
+package commands
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+	"os/signal"
+	"strings"
+	"syscall"
+	"time"
+
+	"github.com/gorilla/websocket"
+	"github.com/spf13/cobra"
+	"golang.org/x/term"
+)
+
+// ShellCmd opens an interactive shell inside a service's running container.
+var ShellCmd = &cobra.Command{
+	Use:   "shell <service-id>",
+	Short: "Interactive shell inside the service container",
+	Args:  cobra.ExactArgs(1),
+	RunE: func(cmd *cobra.Command, args []string) error {
+		serviceID := args[0]
+		cli, err := client()
+		if err != nil {
+			return err
+		}
+		base := strings.TrimRight(cli.BaseURL, "/")
+		wsURL := strings.Replace(base, "http", "ws", 1) + "/services/" + serviceID + "/terminal"
+
+		header := http.Header{}
+		header.Set("Authorization", "Bearer "+cli.Token)
+		conn, _, err := websocket.DefaultDialer.Dial(wsURL, header)
+		if err != nil {
+			return fmt.Errorf("terminal: %w", err)
+		}
+		defer conn.Close()
+
+		fd := int(os.Stdin.Fd())
+		raw := term.IsTerminal(fd)
+		var oldState *term.State
+		if raw {
+			oldState, err = term.MakeRaw(fd)
+			if err == nil {
+				defer term.Restore(fd, oldState)
+			}
+			if w, h, err := term.GetSize(fd); err == nil {
+				_ = conn.WriteJSON(map[string]interface{}{"type": "resize", "cols": w, "rows": h})
+			}
+			// Forward SIGWINCH resizes.
+			sigCh := make(chan os.Signal, 1)
+			signal.Notify(sigCh, syscall.SIGWINCH)
+			defer signal.Stop(sigCh)
+			go func() {
+				for range sigCh {
+					if w, h, err := term.GetSize(fd); err == nil {
+						_ = conn.WriteJSON(map[string]interface{}{"type": "resize", "cols": w, "rows": h})
+					}
+				}
+			}()
+		}
+
+		done := make(chan struct{})
+		// stdin → ws
+		go func() {
+			buf := make([]byte, 4096)
+			for {
+				n, err := os.Stdin.Read(buf)
+				if n > 0 {
+					if werr := conn.WriteJSON(map[string]interface{}{
+						"type": "stdin", "data": string(buf[:n]),
+					}); werr != nil {
+						return
+					}
+				}
+				if err != nil {
+					return
+				}
+			}
+		}()
+		// ws → stdout
+		go func() {
+			defer close(done)
+			for {
+				var frame struct {
+					Type    string `json:"type"`
+					Data    string `json:"data"`
+					Code    int    `json:"code"`
+					Message string `json:"message"`
+				}
+				if err := conn.ReadJSON(&frame); err != nil {
+					return
+				}
+				switch frame.Type {
+				case "stdout":
+					_, _ = os.Stdout.WriteString(frame.Data)
+				case "exit":
+					fmt.Fprintf(os.Stderr, "\r\n[session ended — exit %d]\r\n", frame.Code)
+					return
+				case "error":
+					fmt.Fprintf(os.Stderr, "\r\nerror: %s\r\n", frame.Message)
+					return
+				}
+			}
+		}()
+
+		select {
+		case <-done:
+		case <-time.After(24 * time.Hour):
+		}
+		return nil
+	},
+}