Implement document upload to object storage

[krapfj23]

Summary

Build a storage service that uploads files to the chosen S3-compatible provider.

Blocked by

• Research: evaluate free S3-compatible object storage #71 (object storage research)

Blocks

• Wire object storage into ingest pipeline #73 (wire object storage into ingest pipeline)

Steps

1. Install the SDK for the chosen provider (e.g., supabase Python client, boto3 for S3-compatible)
2. Create backend/app/services/storage.py with functions: upload_file(file_path, bucket, key) -> url and download_file(bucket, key) -> bytes
3. Add storage env vars to cognee_config.py and .env.example
4. Write tests: upload a file → get URL back, download by key → get same content

Acceptance Criteria

• storage.py with upload_file and download_file functions
• File appears in the storage provider's dashboard after upload
• Tests pass for upload + download round-trip
• PR body includes Closes #72