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/api/routes.go

@@ -388,6 +388,9 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 			authed.POST("/deployments/:id/rollback", handleRollbackDeployment)
 			authed.POST("/deployments/:id/cancel", handleCancelDeployment)
 
+			// Interactive terminal — WebSocket → docker exec pty.
+			authed.GET("/services/:id/terminal", handleServiceTerminal)
+
 			// Environment variables — secrets, owner/admin only.
 			authed.GET("/services/:id/variables", handleGetVariables)
 			authed.PUT("/services/:id/variables", handleUpdateVariables)