fix(deploy): use pg_isready without -U to avoid interpolation issue

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

docker/compose.prod.yml

@@ -9,7 +9,7 @@ services:
     volumes:
       - postgres_data:/var/lib/postgresql/data
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER}"]
+      test: ["CMD-SHELL", "pg_isready -h localhost"]
       interval: 10s
       timeout: 5s
       retries: 10