Soft delete a knowledge base and clean up associated vector data.

This endpoint:

1. Verifies KB exists and user has permission, locking the KB row
2. In one transaction, soft deletes all chunks (lightrag_doc_chunks), all documents (knowledge_documents) and the knowledge base (is_deleted=True)
3. Deletes the KB's vectors from Weaviate, best-effort, after the transaction commits: the legacy tenant collections (by workspace) and this tenant's objects in the v2 Doc_kb_<id>_chunks collection
4. Preserves chunk access logs for analytics
5. Leaves S3 files intact (for audit/recovery purposes)

Note: The knowledge base can be restored by setting is_deleted=False, but vector data will need to be re-ingested from the preserved chunks.