Remove volumes on docker compose down to prevent stale credentials

Added -v flag to 'docker compose down' to remove volumes when tearing down
PR preview environments. This ensures PostgreSQL doesn't reuse old volumes
with stale credentials that no longer match updated secrets.

Without this flag, the pr-201_postgres_data volume persists between
deployments, causing authentication failures when secrets are rotated.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: lmcdonough

.github/workflows/deploy-pr-preview.yml

@@ -401,7 +401,7 @@ jobs:
           docker pull ${BACKEND_IMAGE}
 
           echo "🛑 Stopping existing PR-${PR_NUMBER} environment..."
-          docker compose -p ${PROJECT_NAME} -f pr-${PR_NUMBER}-compose.yaml down 2>/dev/null || true
+          docker compose -p ${PROJECT_NAME} -f pr-${PR_NUMBER}-compose.yaml down -v 2>/dev/null || true
 
           echo "🚀 Starting PR preview environment..."
           # Use --env-file to pass environment variables to Docker Compose