fix(docker): remove --frozen-lockfile from prod install

Pruned lockfile may not be compatible with --frozen-lockfile.

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

Author: junzero741

apps/backend/Dockerfile

@@ -39,7 +39,7 @@ RUN corepack enable && corepack prepare pnpm@8.15.5 --activate
 COPY --from=builder /app/dist/apps/backend ./
 
 # Install production dependencies only
-RUN pnpm install --prod --frozen-lockfile
+RUN pnpm install --prod
 
 # Copy Prisma schema + migrations for runtime migrate
 COPY --from=builder /app/apps/backend/prisma ./prisma