feat: implement unified S3-compatible storage system

Consolidates storage backends into a single S3-compatible driver that supports:
- AWS S3 (native)
- Backblaze B2 (S3-compatible API)
- Cloudflare R2 (S3-compatible API)
- MinIO and other S3-compatible services
- Local filesystem for development

This replaces the previous separate B2 driver with a unified approach,
reducing dependencies and complexity while adding support for more services.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

go.sum

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