perf: revert to all-MiniLM-L6-v2, query embed cache, fire-and-forget Cloudinary, pre-warm reranker, reduce top-k

fix: skip reranker build-time cache (1.1 GB OOMs builder), lazy-load at runtime

perf: upgrade embedding to all-mpnet-base-v2, add cross-encoder reranker, LRU cache, enable PII by default

fix models.py: match Neon schema varchar sizes, add email unique constraint