feat: deployment rollback to previous image

- Add previous_image_id column to apps table (migration 006)
- Save current image as previous before deploying new one
- POST /apps/{id}/rollback endpoint with handler
- Rollback stops current container, starts previous image
- Creates deployment record for rollback operations
- Rollback button in app detail UI (only when previous image exists)
- Add btn-warning CSS class for rollback button styling

fixes #71

internal/service/deploy/deploy.go

@@ -49,6 +49,8 @@ var (
 	ErrBuildTimeout = errors.New("build timeout exceeded")
 	// ErrDeployTimeout indicates the deploy phase exceeded the timeout.
 	ErrDeployTimeout = errors.New("deploy timeout exceeded")
+	// ErrNoPreviousImage indicates there is no previous image to rollback to.
+	ErrNoPreviousImage = errors.New("no previous image available for rollback")
 )
 
 // logFlushInterval is how often to flush buffered logs to the database.
@@ -359,6 +361,107 @@ func (svc *Service) Deploy(
 	return svc.runBuildAndDeploy(deployCtx, bgCtx, app, deployment)
 }
 
+// Rollback rolls back an app to its previous image.
+// It stops the current container, starts a new one with the previous image,
+// and creates a deployment record for the rollback.
+func (svc *Service) Rollback(ctx context.Context, app *models.App) error {
+	if !app.PreviousImageID.Valid || app.PreviousImageID.String == "" {
+		return ErrNoPreviousImage
+	}
+
+	// Acquire per-app deployment lock
+	if !svc.tryLockApp(app.ID) {
+		return ErrDeploymentInProgress
+	}
+	defer svc.unlockApp(app.ID)
+
+	bgCtx := context.WithoutCancel(ctx)
+
+	deployment, err := svc.createRollbackDeployment(bgCtx, app)
+	if err != nil {
+		return err
+	}
+
+	return svc.executeRollback(ctx, bgCtx, app, deployment)
+}
+
+// createRollbackDeployment creates a deployment record for a rollback operation.
+func (svc *Service) createRollbackDeployment(
+	ctx context.Context,
+	app *models.App,
+) (*models.Deployment, error) {
+	deployment := models.NewDeployment(svc.db)
+	deployment.AppID = app.ID
+	deployment.Status = models.DeploymentStatusDeploying
+	deployment.ImageID = sql.NullString{String: app.PreviousImageID.String, Valid: true}
+
+	saveErr := deployment.Save(ctx)
+	if saveErr != nil {
+		return nil, fmt.Errorf("failed to create rollback deployment: %w", saveErr)
+	}
+
+	_ = deployment.AppendLog(ctx, "Rolling back to previous image: "+app.PreviousImageID.String)
+
+	return deployment, nil
+}
+
+// executeRollback performs the container swap for a rollback.
+func (svc *Service) executeRollback(
+	ctx context.Context,
+	bgCtx context.Context,
+	app *models.App,
+	deployment *models.Deployment,
+) error {
+	previousImageID := app.PreviousImageID.String
+
+	svc.removeOldContainer(ctx, app, deployment)
+
+	rollbackOpts, err := svc.buildContainerOptions(ctx, app, deployment.ID)
+	if err != nil {
+		svc.failDeployment(bgCtx, app, deployment, err)
+
+		return fmt.Errorf("failed to build container options: %w", err)
+	}
+
+	rollbackOpts.Image = previousImageID
+
+	containerID, err := svc.docker.CreateContainer(ctx, rollbackOpts)
+	if err != nil {
+		svc.failDeployment(bgCtx, app, deployment, fmt.Errorf("failed to create rollback container: %w", err))
+
+		return fmt.Errorf("failed to create rollback container: %w", err)
+	}
+
+	deployment.ContainerID = sql.NullString{String: containerID, Valid: true}
+	_ = deployment.AppendLog(bgCtx, "Rollback container created: "+containerID)
+
+	startErr := svc.docker.StartContainer(ctx, containerID)
+	if startErr != nil {
+		svc.failDeployment(bgCtx, app, deployment, fmt.Errorf("failed to start rollback container: %w", startErr))
+
+		return fmt.Errorf("failed to start rollback container: %w", startErr)
+	}
+
+	_ = deployment.AppendLog(bgCtx, "Rollback container started")
+
+	currentImageID := app.ImageID
+	app.ImageID = sql.NullString{String: previousImageID, Valid: true}
+	app.PreviousImageID = currentImageID
+	app.Status = models.AppStatusRunning
+
+	saveErr := app.Save(bgCtx)
+	if saveErr != nil {
+		return fmt.Errorf("failed to update app after rollback: %w", saveErr)
+	}
+
+	_ = deployment.MarkFinished(bgCtx, models.DeploymentStatusSuccess)
+	_ = deployment.AppendLog(bgCtx, "Rollback complete")
+
+	svc.log.Info("rollback completed", "app", app.Name, "image", previousImageID)
+
+	return nil
+}
+
 // runBuildAndDeploy executes the build and deploy phases, handling cancellation.
 func (svc *Service) runBuildAndDeploy(
 	deployCtx context.Context,
@@ -390,6 +493,11 @@ func (svc *Service) runBuildAndDeploy(
 		return err
 	}
 
+	// Save current image as previous before updating to new one
+	if app.ImageID.Valid && app.ImageID.String != "" {
+		app.PreviousImageID = app.ImageID
+	}
+
 	err = svc.updateAppRunning(bgCtx, app, imageID)
 	if err != nil {
 		return err