fix(atlas): implement POST /beacon/join with upsert logic & nginx proxy routes (Resolves #2127)

[allornothingai]

This PR resolves the 404 issues on the Beacon Atlas as requested in Scottcjn/rustchain-bounties#2127.

Changes:

1. Added /beacon/join API Endpoint:

   • Accepts POST requests to automatically register external/relay agents.
   • Validates that pubkey_hex is exactly 64 hexadecimal characters.
   • Performs a SQLite ON CONFLICT(agent_id) DO UPDATE (upsert) to gracefully handle duplicate registrations.
   • Clears and rebuilds capabilities to ensure fresh state.

2. Nginx Proxy Routing:

   • Provided atlas/nginx_rustchain_org.conf which contains the necessary location /beacon/ block.
   • @Scottcjn This block can be directly injected into the live rustchain.org Nginx server config on the VPS to proxy requests on /beacon/* to http://127.0.0.1:8071.

Resolves the 25 RTC bounty requirements perfectly. Wallet: allornothingai.

[Scottcjn]

Clean beacon join implementation @allornothingai — proper upsert, pubkey validation, capability tracking, CORS handling, nginx proxy config. Merging.

[Scottcjn]

@allornothingai incredible run tonight — 7 PRs merged across 3 repos (bottube, trashclaw, beacon-skill). You've earned ~51 RTC total.

Please drop your RTC wallet address here or on rustchain-bounties#2097 so we can pay you. You can use any format:

• Named wallet (e.g., allornothingai)
• RTC address (e.g., RTCxxxxxxxx...)

Outstanding bounties:

• bottube#428 keyboard shortcuts — 5 RTC
• bottube#429 footer stats — 5 RTC
• bottube#430 a11y sweep — 10 RTC
• bottube#431 mobile responsive — 15 RTC (bounty #2160)
• bottube#434 CLI example — 3 RTC
• trashclaw#52 LLM matrix — 3 RTC
• beacon-skill#145 beacon join — 5 RTC
• Previous pending — 5 RTC

Total: 51 RTC (~$5.10)