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.

backend/scripts/jenkins-alembic-e2e.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+# Ejecutado solo desde Jenkins E2E: docker compose run --entrypoint /bin/sh … backend /app/scripts/jenkins-alembic-e2e.sh
+# (evita pasar cadenas con ';' a -c: Groovy/CI rompe comillas y parte el comando)
+set -eux
+test -f /app/alembic.ini
+cd /app
+exec alembic -c /app/alembic.ini upgrade head