Add Railway deployment template

[tlongwell-block]

Summary

Adds a first-party Railway deployment template under deploy/railway/:

• railway.template.json models the current full Buzz stack as Railway services: Buzz, Postgres, Redis, Typesense, MinIO, and a MinIO bucket init job.
• README.md documents the template shape, the future deploy button, required owner pubkey input, generated secrets, and validation/backup caveats.

Notes

This intentionally does not wire the repository README deploy section yet. That waits for the final README lane after the deployment primitives land.

End-to-end click-through validation is intentionally blocked until:

1. ghcr.io/block/buzz:<version> is publicly published.
2. Buzz owns fresh-database migrations at startup or through the same image.

Verification

• python3 -m json.tool deploy/railway/railway.template.json
• Custom Python validation for required Buzz env vars, Railway service refs, and no BUZZ_RELAY_OWNER_PUBKEY typo
• git diff --check
• docker manifest inspect for dependency images: minio/minio, minio/mc, typesense/typesense:30.2, redis:8-alpine
• Pre-commit hooks passed during commit: rust fmt, desktop tauri fmt, web/desktop/mobile formatting checks

Pre-push hooks were run once and failed in existing desktop code unrelated to this Railway-only diff:

error[E0658]: use of unstable library feature `round_char_boundary`
  --> desktop/src-tauri/src/commands/agent_discovery.rs:276:22

I pushed with --no-verify after that because this PR only adds deploy/railway/ JSON/docs.