Refactor Alembic command in Jenkinsfile for improved execution

- Updated the Alembic upgrade command to use the direct binary path instead of Python, addressing issues with command parsing in Docker.
- Enhanced comments to clarify the reasoning behind the changes and their implications for E2E testing.

Jenkinsfile

@@ -284,8 +284,9 @@ pipeline {
             e2e_compose_fail_logs
             exit 1
           fi
-          # Nunca usar "python3 -m alembic -c": el -c es ambigüo; Alembic recibe un config vacío (No 'script_location')
-          if ! e2e_compose run --rm --no-deps --entrypoint python3 backend -m alembic --config /app/alembic.ini upgrade head; then
+          # Binario /usr/local/bin/alembic: sin python3 -m (el -c rompe; log con -c = build sin el último commit)
+          # --config=/ruta evita un token que empiece con - (algunos compose run lo mezclan con sus flags)
+          if ! e2e_compose run --rm --no-deps --entrypoint /usr/local/bin/alembic backend --config=/app/alembic.ini upgrade head; then
             echo "ERROR: falló alembic upgrade head (job previo al stack completo)"
             e2e_compose_fail_logs
             exit 1