Delete Knowledge Base

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

This endpoint:

1. Verifies KB exists and user has permission
2. Soft deletes all chunks (sets is_deleted=True, deleted_at, deleted_by in lightrag_doc_chunks)
3. Deletes all vectors from Weaviate (by workspace filter)
4. Soft deletes all documents (sets is_deleted=True, deleted_at, deleted_by in knowledge_documents)
5. Soft deletes the knowledge base (sets is_deleted=True)
6. Preserves chunk access logs for analytics
7. 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.