fix: mount volume at correct path so database survives updates

WORKDIR is /app/backend so the DB lands at /app/backend/data/parks.db,
but the volume was mounted at /app/data — a different directory. The DB
lived in the container's ephemeral layer and was wiped on every pull.

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

docker-compose.yml

@@ -14,7 +14,7 @@ services:
     ports:
       - "3001:3001"
     volumes:
-      - park_data:/app/data
+      - park_data:/app/backend/data
     environment:
       - NODE_ENV=production
       - TZ=America/New_York