feat(relay): plan-based gating, KV error logging, and request logging

[iceteaSA]

Worker script improvements:

• Plan-based gating: WebSocket transport requires paid plan (RELAY_PLAN=paid), returns 403 on free
• KV error logging: Non-429/403 upstream errors logged to KV with 7-day TTL for post-mortem debugging
• Request logging: HTTP and WebSocket requests logged on paid plan only
• GET health endpoint: Returns plan info and available transports

All changes are within the WORKER_SCRIPT constant — no client-side relay code modified.

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Summary by cubic

Adds plan-based gating to the relay worker and improves observability with KV-backed error logging, request logs, and a health endpoint. All changes are scoped to WORKER_SCRIPT; no client relay code touched.

• New Features
  • WebSocket transport requires paid plan (RELAY_PLAN=paid); returns 403 on free.
  • Non-429/403 upstream errors are logged to RELAY_STATE KV with a 7-day TTL (includes response preview and headers).
  • Request logging for HTTP and WebSocket is enabled only on the paid plan.
  • GET health endpoint returns status, plan, and available transports (HTTP on free; HTTP + WebSocket on paid).

^{Written for commit 5e33a83. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

1 issue found across 2 files

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