Wire object storage into ingest pipeline

[krapfj23]

Summary

After cognify, persist the original document to object storage and return the URL.

Blocked by

• Implement document upload to object storage #72 (implement document upload to object storage)

Blocks

Nothing

Steps

1. Open backend/app/services/ingest.py
2. After cognee.cognify() completes, upload the original file to object storage using storage.upload_file()
3. Store the returned object URL in the document response (add file_url field to UploadResponse)
4. Ensure temp file in /tmp/cognee_uploads/ is still deleted after upload to storage
5. Write test: upload document, verify file_url is returned and points to a valid object

Acceptance Criteria

• Uploaded documents are persisted to object storage
• file_url returned in upload response
• Temp files cleaned up after upload
• End-to-end: upload → file in storage → search still works
• PR body includes Closes #73