Refactor Alembic command in Jenkinsfile for clarity and correctness

- Updated the Alembic upgrade command to use the `--config` option instead of `-c`, addressing ambiguity in configuration handling.
- Revised comments to enhance understanding of the command's context and implications during E2E testing.

Jenkinsfile

@@ -284,8 +284,8 @@ pipeline {
             e2e_compose_fail_logs
             exit 1
           fi
-          # Sin sh -c ni .sh bajo /app: sin comillas frágiles y sin depender de un fichero no montado aún (push)
-          if ! e2e_compose run --rm --no-deps --entrypoint python3 backend -m alembic -c /app/alembic.ini upgrade head; then
+          # 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
             echo "ERROR: falló alembic upgrade head (job previo al stack completo)"
             e2e_compose_fail_logs
             exit 1