fix(docker): resolver DATABASE_URL localhost→db no container automaticamente

- docker-entrypoint.sh: substituir @localhost: por @db: via sed antes das
  migrations e do Next.js subirem — transparente para o usuário
- docker-compose.yml: adicionar valor padrão para DATABASE_URL para
  permitir subir sem .env configurado

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -81,7 +81,8 @@ services:
       NODE_ENV: production
 
       # Banco local: use host "db" | Banco remoto: URL completa do provider
-      DATABASE_URL: ${DATABASE_URL}
+      # O entrypoint converte automaticamente "localhost" → "db" se necessário
+      DATABASE_URL: ${DATABASE_URL:-postgresql://openmonetis:openmonetis_dev_password@db:5432/openmonetis_db}
 
       BETTER_AUTH_SECRET: ${BETTER_AUTH_SECRET}
       BETTER_AUTH_URL: ${BETTER_AUTH_URL:-http://localhost:3000}