- compose.yml: add postgres service with profiles, healthcheck, pg_isready
- compose.yml: add DATABASE_URL env var and conditional depends_on (required: false)
- compose.yml: add postgres-data volume; default deploy remains SQLite-only
- compose.dev.yml: add postgres service with port 5432 exposed for local dev
- compose.dev.yml: add DATABASE_URL env var and conditional depends_on
- pkg/diunwebhook/postgres_test.go: build-tagged NewTestPostgresServer helper
