#672 RAG embedding cache moved to PostgreSQL

Fixed RAG embedding cache that was causing hundreds of /embedding requests. Changed from file-based cache (/tmp/highway-rag-cache) to PostgreSQL table (rag_embedding_cache) with TTL. Each container was computing embeddings separately because /tmp is container-local. Now first container computes and saves to DB, others load from cache instantly. Migration: 01KD7969WDSZP28E6RTNZ9MHQ4_rag_embedding_cache.sql