List live, reachable vector stores

Returns the subset of [qdrant, weaviate, pgvector, milvus, chroma] whose Vault secret is present AND whose service responds to a probe. Drives the document-tap pipeline wizard’s store picker. Secret presence alone isn’t sufficient — the dev stack seeds placeholder secrets for every store, so this endpoint must actually probe each one (2s timeout per store, ~10s worst case if everything is unreachable).