Refactor Alembic command execution in Jenkinsfile to use script

- Updated the Alembic upgrade command to execute a script instead of using a variable, addressing issues with command execution in the Docker environment.
- Revised comments to clarify the changes and their implications for the command context during E2E testing.

Jenkinsfile

@@ -284,10 +284,8 @@ pipeline {
             e2e_compose_fail_logs
             exit 1
           fi
-          # Cuerpo de -c en variable: las comillas dobles inline en "if ! e2e_compose run ..." se colaban en
-          # el shell de Jenkins y -c quedaba solo con "set"; el resto se ejecutaba en el host (no existe /app)
-          E2E_ALEMBIC_CMD='set -eux; test -f /app/alembic.ini; cd /app; exec alembic -c /app/alembic.ini upgrade head'
-          if ! e2e_compose run --rm --no-deps --entrypoint /bin/sh backend -c "$E2E_ALEMBIC_CMD"; then
+          # Script en el repo: Groovy/CI mutila comillas de -c o asignación VAR='…;…' (mismo síntoma: test en host)
+          if ! e2e_compose run --rm --no-deps --entrypoint /bin/sh backend /app/scripts/jenkins-alembic-e2e.sh; then
             echo "ERROR: falló alembic upgrade head (job previo al stack completo)"
             e2e_compose_fail_logs
             exit 1