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>

vercel.json

@@ -1,5 +1,9 @@
 {
-  "$schema": "https://openapi.vercel.sh/vercel.json",
-  "bunVersion": "1",
-  "installCommand": "bun install"
+  "buildCommand": "bun install",
+  "outputDirectory": "dist",
+  "functions": {
+    "src/index.ts": {
+      "runtime": "bun@1"
+    }
+  }
 }