feat(qdrant): optimization — payload indexes, HNSW tuning, search params (v1)

Inspection findings:
- _ensure_collection() created collections with bare VectorParams (no HNSW/optimizer config)
- _do_search() had no SearchParams — used Qdrant defaults (ef often ~100, no indexed_only)
- No payload index management at all — filtered searches scanned unindexed fields every time
- collection_info() returned minimal data — impossible to inspect production state
- No way to create/ensure payload indexes via the API

Changes — qdrant/main.py:
- Add SEARCH_HNSW_EF env var (default 128, above Qdrant default for better recall)
- _ensure_collection(): configure HnswConfigDiff(m=16, ef_construct=200, on_disk=False)
  and OptimizersConfigDiff(indexing_threshold=20000, default_segment_number=4) on creation
- _do_search(): use SearchParams(hnsw_ef, exact, indexed_only) on every query
- SearchUrlRequest + SearchVectorRequest: expose hnsw_ef, exact, indexed_only per request
- collection_info(): expand to full HNSW/optimizer/quantization/segment/payload_schema detail
- GET  /collections/{name}/indexes     — list all payload indexes
- POST /collections/{name}/indexes     — create a single payload index
- POST /collections/{name}/ensure-indexes — idempotent bulk index creation (skip existing)
- POST /collections/{name}/configure   — apply HNSW/optimizer changes to existing collections

Changes — gateway/main.py:
- Expose the 4 new qdrant-svc endpoints under /vectors/collections/{name}/...

Changes — docker-compose.yml:
- Add SEARCH_HNSW_EF=128 to qdrant-svc environment

Critical usage note for existing collections:
  After deploying, call POST /vectors/collections/images/ensure-indexes with the
  payload fields actually used in filter_metadata (is_public, category_id, etc.)
  to add missing indexes. This is the highest-impact single action for filtered search.

docker-compose.yml

@@ -77,6 +77,7 @@ services:
       - CLIP_URL=http://clip:8000
       - COLLECTION_NAME=images
       - VECTOR_DIM=512
+      - SEARCH_HNSW_EF=128
     depends_on:
       qdrant:
         condition: service_healthy