Why:
- The chunks collection is now created by an explicit deployment step
(qdrant_bootstrap), not at startup, which means a process can boot against
a healthy Qdrant that has no collection at all. /readyz's previous check
only called get_collections(), so it reported ready in that state — the
misconfiguration stayed invisible until the first upload failed with a 502
after already paying for the MinIO write and embedding round trips.
Changes:
- ping_qdrant() now checks collection_exists(collection) instead of just
reachability.
