Deploy target: Koyeb

[frostyhand]

We ship Render, Fly.io, and Railway configs; Koyeb is a natural fourth (Docker-native, free tier, one-click buttons).

Acceptance:

• A config or button-ready setup for deploying ghcr.io/axiorank/gateway (or Dockerfile build) on Koyeb, with the /data volume story documented (signing key persistence)
• README Deploy section row
• Verified against Koyeb's current docs; note anything that does not map (for example if persistent volumes are limited, say so honestly and document the receipt-key tradeoff)