Rerank passages by relevance to a query via Cloudflare Workers AI (@cf/baai/bge-reranker-base). Send {query, documents[] (≤50, each ≤4000 chars), topK?}; returns [{index, score}] best-first — the missing link after embed/chunk, keeping the top chunks before a costly LLM call. Score is the model's relevance score (higher = better), named as it only compares within this model. Unavailable binding → 503, unbilled.