Fix two critical backend bugs found during integration testing

- versions.py: eagerly load public_assets in create_branch and append_patches_to_version refresh queries; missing selectinload caused MissingGreenlet 500 on every branch creation
- documents.py: null root_version_id before cascade delete to break the circular FK (cv_documents → cv_versions → cv_documents), which was causing IntegrityError on every document deletion

https://claude.ai/code/session_017HGM9VPptZG52asT5pbL6Y

apps/backend/fastapi/app/services/documents.py

@@ -99,6 +99,9 @@ async def delete_document(
         await session.execute(
             delete(PublicAsset).where(PublicAsset.version_id.in_(version_ids))
         )
+    # Null root_version_id to break the circular FK before cascade
+    doc.root_version_id = None
+    await session.flush()
     await session.delete(doc)
     await session.commit()
     for key in artifact_keys: