fix(deploy): move POSTGRES_PASSWORD default into .env.example

Plain ${POSTGRES_PASSWORD} in compose; default 'steward' lives in .env.example
where it's visible and editable rather than buried in compose YAML.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.env.example

@@ -9,4 +9,4 @@ STEWARD_SMTP__PASSWORD=
 # Password for the bundled Postgres; the steward service builds its
 # STEWARD_DATABASE_URL from it. Leave STEWARD_SECRET_KEY above unset to let the
 # app generate + persist one on the /data volume.
-POSTGRES_PASSWORD=change-me
+POSTGRES_PASSWORD=steward