fix(docker): align database config between docker-compose and .env

- Move POSTGRES_USER, POSTGRES_PASSWORD, POSTGRES_DB to .env
- Update docker-compose.yml to reference env vars instead of hardcoding
- Fix db healthcheck to use correct credentials from .env
- Update .env.example with matching placeholder values
- Update DATABASE_URL to use variable interpolation

.env.example

@@ -7,9 +7,9 @@ FLASK_DEBUG=1
 # ═════════════════════════════════════════════════════════════════════════════
 COMPOSE_PROJECT_NAME=authy2
 FLASK_ENV=production
-POSTGRES_USER=authy2
-POSTGRES_PASSWORD=changeme-in-production
-POSTGRES_DB=authy2
+POSTGRES_USER=appuser
+POSTGRES_PASSWORD=change-me-in-production
+POSTGRES_DB=secuird
 DATABASE_URL=postgresql://${POSTGRES_USER}:${POSTGRES_PASSWORD}@db:5432/${POSTGRES_DB}
 SQLALCHEMY_DATABASE_URI=${DATABASE_URL}
 REDIS_URL=redis://redis:6379/0