fix: guardar programa de obra (GET fallaba por agregados SQL anidados)

<!-- CURSOR_AGENT_PR_BODY_BEGIN -->
## Problema

La preview del Excel por partida ya funciona. Al pulsar **Importar** el programa sí se guarda (`Programa importado`), pero al recargar `GET /v1/projects/:id/work-program` aparece:

`Error inesperado en fn_work_program_get (sqlstate=42803): aggregate function calls cannot be nested`

Postgres no permite `jsonb_agg(... SUM(...))` en el mismo SELECT. Eso arma la curva semanal después del import.

## Cambio

Changeset `036`: `SUM` va en una subquery y `jsonb_agg` queda afuera. El import no cambia.

## Cómo probar

1. Aplicar migraciones Liquibase (changeset 036).
2. En una obra con presupuesto, importar el Excel por partida (el de 8 partidas / 9 semanas).
3. Tras **Importar**, la pantalla debe mostrar el programa (semanas y curva), sin el toast rojo.
4. Actualizar / volver a entrar a Programa de obra: sigue cargando.

<!-- CURSOR_AGENT_PR_BODY_END -->

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db/core/changelog-master.xml

@@ -45,5 +45,6 @@
   <include file="changesets/033-catalog-suppliers-units.sql" relativeToChangelogFile="true"/>
   <include file="changesets/034-fn-expense-update-reclassify.sql" relativeToChangelogFile="true"/>
   <include file="changesets/035-warehouse-materials-catalog.sql" relativeToChangelogFile="true"/>
+  <include file="changesets/036-fn-work-program-get-curve.sql" relativeToChangelogFile="true"/>
 
 </databaseChangeLog>

db/core/changesets/036-fn-work-program-get-curve.sql

@@ -0,0 +1,92 @@
+--liquibase formatted sql
+-- PANELS · core · fn_work_program_get: jsonb_agg no puede anidar SUM (42803)
+
+--changeset panel:core-036-fn-work-program-get-curve splitStatements:false
+CREATE OR REPLACE FUNCTION core.fn_work_program_get(payload jsonb)
+RETURNS jsonb
+LANGUAGE plpgsql
+SECURITY INVOKER
+SET search_path = core
+AS $$
+DECLARE
+  v_project_id bigint := NULLIF(payload->>'project_id', '')::bigint;
+  v_program jsonb;
+  v_periods jsonb;
+  v_budget_total numeric(14,2);
+  v_scheduled jsonb;
+  v_cumulative numeric := 0;
+  v_row jsonb;
+  v_arr jsonb := '[]'::jsonb;
+  v_pct_period numeric;
+  v_pct_cum numeric;
+BEGIN
+  IF v_project_id IS NULL THEN
+    RETURN core.rpc_err('VALIDATION', 'fn_work_program_get: project_id es obligatorio',
+      jsonb_build_object('fn', 'fn_work_program_get'));
+  END IF;
+
+  SELECT to_jsonb(wp) INTO v_program
+  FROM work_programs wp WHERE wp.project_id = v_project_id;
+
+  IF v_program IS NULL THEN
+    SELECT COALESCE(SUM(amount), 0) INTO v_budget_total FROM budget_items WHERE project_id = v_project_id;
+    RETURN core.rpc_ok(
+      jsonb_build_object(
+        'program', NULL,
+        'periods', '[]'::jsonb,
+        'budget_subtotal', v_budget_total,
+        'curve', '[]'::jsonb
+      ),
+      format('Proyecto %s sin programa de obra', v_project_id),
+      jsonb_build_object('fn', 'fn_work_program_get', 'project_id', v_project_id)
+    );
+  END IF;
+
+  SELECT COALESCE(jsonb_agg(to_jsonb(p) ORDER BY p.period_index), '[]'::jsonb) INTO v_periods
+  FROM work_program_periods p WHERE p.program_id = (v_program->>'id')::bigint;
+
+  SELECT COALESCE(SUM(amount), 0) INTO v_budget_total FROM budget_items WHERE project_id = v_project_id;
+
+  -- SUM va en la subquery; jsonb_agg afuera. Postgres no permite agregados anidados
+  -- (sqlstate 42803) y esto petaba el GET justo después de importar por partida.
+  SELECT COALESCE(jsonb_agg(to_jsonb(x) ORDER BY x.period_index), '[]'::jsonb)
+  INTO v_scheduled
+  FROM (
+    SELECT
+      pr.period_index,
+      pr.label,
+      round(COALESCE(SUM(pa.amount), 0), 2) AS amount
+    FROM work_program_periods pr
+    LEFT JOIN work_program_partida_amounts pa ON pa.period_id = pr.id
+    WHERE pr.program_id = (v_program->>'id')::bigint
+    GROUP BY pr.period_index, pr.label
+  ) x;
+
+  FOR v_row IN SELECT value FROM jsonb_array_elements(v_scheduled) LOOP
+    v_pct_period := CASE WHEN v_budget_total > 0
+      THEN round((v_row.value->>'amount')::numeric / v_budget_total, 6) ELSE 0 END;
+    v_cumulative := v_cumulative + (v_row.value->>'amount')::numeric;
+    v_pct_cum := CASE WHEN v_budget_total > 0 THEN round(v_cumulative / v_budget_total, 6) ELSE 0 END;
+    v_arr := v_arr || jsonb_build_array(jsonb_build_object(
+      'period_index', v_row.value->'period_index',
+      'label', v_row.value->'label',
+      'amount', v_row.value->'amount',
+      'pct_period', v_pct_period,
+      'pct_cumulative', v_pct_cum
+    ));
+  END LOOP;
+
+  RETURN core.rpc_ok(
+    jsonb_build_object(
+      'program', v_program,
+      'periods', v_periods,
+      'budget_subtotal', v_budget_total,
+      'curve', v_arr
+    ),
+    format('Programa de obra del proyecto %s', v_project_id),
+    jsonb_build_object('fn', 'fn_work_program_get', 'project_id', v_project_id)
+  );
+EXCEPTION WHEN OTHERS THEN
+  RETURN core.rpc_from_exception('fn_work_program_get', SQLSTATE, SQLERRM);
+END;
+$$;