feat: migrate from Next.js to pure Bun relay proxy

- Replace Next.js app router with standalone Bun.serve() entry point
- Add middleware stack: rate limiter, body limiter, structured logger, SSRF protection
- Add WebSocket bidirectional relay via x-relay-target header
- Implement error classification (DNS→502, timeouts→504, SSRF→403, rate→429)
- Remove all Next.js dependencies and config files
- Update deploy workflow, tsconfig, wrangler config for Bun deployment

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/middleware/index.ts

@@ -0,0 +1,5 @@
+export { checkBodySize, getMaxBodySize, setMaxBodySize } from "./body-limiter";
+export type { RelayLogEvent } from "./logger";
+export { createRequestLogger, logRelayEvent } from "./logger";
+export type { RateLimiter, RateLimiterOptions } from "./rate-limiter";
+export { createRateLimiter } from "./rate-limiter";