fix: move HF_HUB_OFFLINE after reranker pre-download

fix: set HF_HOME to /app/.cache so user 1000 can read baked reranker weights

feat: migrate backend to HF Spaces Docker (16GB RAM free tier)

fix: Switch reranker to TinyBERT-L-2-v2 (~17MB) to prevent OOM on web queries

fix: Install CPU-only torch to prevent OOM on Render 512MB free tier

fix: Pre-download cross-encoder at build time to prevent Render port scan timeout

frontend changes

Add Dockerfile and Render config