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/server/routes.go

@@ -76,6 +76,7 @@ func (s *Server) SetupRoutes() {
 		r.Get("/apps/{id}/container-logs", s.handlers.HandleContainerLogsAPI())
 		r.Get("/apps/{id}/status", s.handlers.HandleAppStatusAPI())
 		r.Get("/apps/{id}/recent-deployments", s.handlers.HandleRecentDeploymentsAPI())
+		r.Post("/apps/{id}/rollback", s.handlers.HandleAppRollback())
 		r.Post("/apps/{id}/restart", s.handlers.HandleAppRestart())
 		r.Post("/apps/{id}/stop", s.handlers.HandleAppStop())
 		r.Post("/apps/{id}/start", s.handlers.HandleAppStart())