feat: Add mobile-friendly /inbox/{id} endpoint with 2-phase ack

[SeverinAlexB]

Replaces #1

The current main branch is a copy of the pubky-core http-relay which was used for testing before. The goal is to extract this library to clean up the core repo.

This branch tries to solve #2 with a new inbox model + caching of the values for 5min. It also adds features to make the code more mature for a production environment.

Also adds a demo that is auto deployed on a merge on master and is available here: https://pubky.github.io/http-relay/

Summary

• Add /inbox/{id} endpoint with response caching and shorter timeout to support mobile clients that may be backgrounded or killed during requests
• Add security limits: max body size, max pending requests, max cached entries, max channel ID length
• Refactor single-file implementation into modular structure under src/http_relay/
• Add comprehensive CLI options for all configurable parameters
• Add OpenAPI specification and improved documentation

Code Quality

This repo is fully vibe coded. I double checked the important areas but let me know if you want me to unslop things.