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>

Dockerfile

@@ -51,8 +51,8 @@ COPY --chown=nextjs:nodejs backend/package.json ./backend/package.json
 COPY --chown=nextjs:nodejs backend/tsconfig.json ./backend/tsconfig.json
 COPY --chown=nextjs:nodejs lib ./lib
 
-RUN mkdir -p /app/data && chown nextjs:nodejs /app/data
-VOLUME ["/app/data"]
+RUN mkdir -p /app/backend/data && chown nextjs:nodejs /app/backend/data
+VOLUME ["/app/backend/data"]
 
 USER nextjs
 EXPOSE 3001