perf(relay): defer KV state write, add health endpoint, harden POST handler

[iceteaSA]

Summary

Four improvements to the relay worker script:

1. Deferred KV state write (~4ms P50 improvement)

prepareUpstream previously awaited the KV writeState call before returning the body. Now the write is fire-and-forget (.catch(() => {})) and deferred via ctx.waitUntil. Response streaming starts immediately — the state is persisted in the background.

2. Heartbeat interval 5s → 15s

Reduces WebSocket keepalive overhead by 3x. The CF Workers runtime keeps connections alive independently; the heartbeat only needs to prevent idle timeouts on intermediate proxies, which typically have 30-60s thresholds.

3. GET / health endpoint

Returns JSON with relay status and available transports instead of plain "ok":

{"status":"ok","transports":["http","websocket"]}

4. POST handler try/catch → 502

Wraps the HTTP POST handler in a try/catch that returns structured JSON 502 instead of raw Cloudflare error pages on unexpected failures. Clients can parse the error and fall back to direct API calls.

Testing

All 214 tests pass. Changes are in the embedded WORKER_SCRIPT template string — no client-side behavior changes.