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/handlers/app.go

@@ -380,6 +380,30 @@ func (h *Handlers) HandleCancelDeploy() http.HandlerFunc {
 	}
 }
 
+// HandleAppRollback handles rolling back to the previous deployment image.
+func (h *Handlers) HandleAppRollback() http.HandlerFunc {
+	return func(writer http.ResponseWriter, request *http.Request) {
+		appID := chi.URLParam(request, "id")
+
+		application, findErr := models.FindApp(request.Context(), h.db, appID)
+		if findErr != nil || application == nil {
+			http.NotFound(writer, request)
+
+			return
+		}
+
+		rollbackErr := h.deploy.Rollback(request.Context(), application)
+		if rollbackErr != nil {
+			h.log.Error("rollback failed", "error", rollbackErr, "app", application.Name)
+			http.Redirect(writer, request, "/apps/"+application.ID, http.StatusSeeOther)
+
+			return
+		}
+
+		http.Redirect(writer, request, "/apps/"+application.ID+"?success=rolledback", http.StatusSeeOther)
+	}
+}
+
 // HandleAppDeployments returns the deployments history handler.
 func (h *Handlers) HandleAppDeployments() http.HandlerFunc {
 	tmpl := templates.GetParsed()