fix(rutinas): castear NULL a INTEGER en migración 003

Postgres tipa el NULL pelado del SELECT como text y rechaza insertarlo
en routine_task.coins_reward (integer). H2 lo toleraba; Postgres no.
Se castea con CAST(NULL AS INTEGER). Validado arrancando contra
Postgres 16 real: los 5 changesets de 003 aplican y la app arranca.

backend/src/main/resources/db/changelog/changes/003-routine-catalog.yaml

@@ -38,7 +38,8 @@ databaseChangeLog:
         - sql:
             sql: >-
               INSERT INTO routine_task (family_id, label_es, label_ca, icon, color, coins_reward)
-              SELECT DISTINCT c.family_id, ar.label_es, ar.label_ca, ar.icon, ar.color, NULL
+              SELECT DISTINCT c.family_id, ar.label_es, ar.label_ca, ar.icon, ar.color,
+                     CAST(NULL AS INTEGER)
               FROM afternoon_routine ar
               JOIN child c ON c.id = ar.child_id;
       rollback: