relay: migrate from Fastify to Bun.serve() native

[YairEtzion]

Summary

• Replace Fastify v5 + @fastify/websocket + ws with Bun's native Bun.serve() HTTP and WebSocket server
• Drop 62 transitive dependencies from the relay package (fastify, @fastify/websocket, @fastify/rate-limit, ws, and all their transitive deps)
• Update Dockerfile.relay to use oven/bun:1.3-slim as runtime (Node.js kept for build stage only)
• Switch relay tests from vitest to bun:test since the server requires Bun runtime

Why

• Anthropic acquired Bun (Dec 2025); Claude Code ships as a Bun executable
• The relay is a thin WebSocket message router — Fastify added framework overhead with no benefit
• Bun's native WebSocket is faster than the ws library polyfill
• Smaller Docker image, faster cold starts on Cloud Run

What changed

File	Change
packages/relay/src/server.ts	Full rewrite: Fastify → Bun.serve() with native WebSocket handlers
packages/relay/src/session.ts	Types updated for ServerWebSocket<WebSocketData>
packages/relay/src/start.ts	Simplified (Bun.serve returns synchronously)
packages/relay/src/index.ts	Added WebSocketData type export
packages/relay/src/__tests__/	vitest → bun:test
packages/relay/package.json	Removed 5 deps, added @types/bun, test script uses bun test
Dockerfile.relay	Node.js build stage + oven/bun:1.3-slim runtime stage
pnpm-lock.yaml	-409 lines (62 packages removed)

Test plan

• bun test — all 5 relay tests pass (handshake integration + SAS computation)
• Local Docker build and run — health endpoint + WebSocket verified
• Deployed to Cloud Run (amesh-relay) — health + WebSocket verified
• Custom domain relay.authmesh.dev — confirmed working
• pnpm build — all packages build successfully