Store original documents for download after ingestion

Persist uploaded files to {data_dir}/documents/{content_hash}{ext} after
successful ingestion. Add GET /documents/{id}/file endpoint for retrieval,
delete stored files on document deletion, and add `kb export` client command.
Includes schema migration, tests, and spec updates.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

engine/compose.rocm.yaml

@@ -18,4 +18,5 @@ services:
       - KB_INGEST_DEVICE=${KB_INGEST_DEVICE:-auto}
       - KB_API_KEY=${KB_API_KEY:-}
       - KB_SEARCH_THRESHOLD=${KB_SEARCH_THRESHOLD:-0.01}
+      - HF_HUB_OFFLINE=${HF_HUB_OFFLINE:-}
     restart: unless-stopped