Hybrid search scoped to this vault's namespace.

Returns embedded vault pages as ranked results with citations. The vault namespace was populated by the existing fs_sync_workflow mirroring every write into TurboPuffer (see routes/v2/fs.py:294-313).

Telemetry: emits both store_api_activity (for the user-facing activity feed) and store_retrieval_log (for the training-data pipeline) so vault searches show up alongside every other retrieval call. Two retrieval logs are written for the two-tier strategy: a vector log for the TurboPuffer path, and a regex log if the PG grep fallback fires.