Research: evaluate free S3-compatible object storage

[krapfj23]

Summary

Compare free/cheap S3-compatible storage providers and recommend one for storing uploaded documents.

Blocked by

Nothing — this is a starting ticket.

Blocks

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

Steps

1. Compare: Supabase Storage (1GB free, already in stack), Cloudflare R2 (10GB free, no egress), Backblaze B2 (10GB free), MinIO (self-hosted)
2. Evaluate on: free tier size, S3 API compatibility, Python SDK support, ease of setup, egress costs
3. Test uploading/downloading a file from the top 2 candidates using Python
4. Consider: since Supabase is already in the stack, is Supabase Storage the simplest choice?

Acceptance Criteria

• docs/object-storage-research.md exists with comparison table
• At least 2 providers tested with real uploads
• Clear recommendation with reasoning
• PR body includes Closes #71