Add S3 / Cloudflare R2 media upload endpoint in server/index.js⁠

[ProofTubeProtocol]

Overview

We need to expand server/index.js to handle video file uploads and multipart chunking for decentralized delivery.

Core Requirements

• Implement multipart upload endpoints for video/audio file ingestion.
• Integrate AWS S3 / Cloudflare R2 API storage adapters.
• Return structured HLS manifest paths (.m3u8) and segment hashes upon upload completion.

Resources & Specs

Refer to Section 2.1 in WHITEPAPER.md for architecture details.