fix: critical bugs, serverless stability, and security hardening

Bug Fixes:
- Fix rate limiter API mismatch: check() -> checkAsync() in index.ts, worker.ts, api/relay.ts
- Fix WebSocket SSRF silent drop: return error Response instead of undefined
- Fix isDevMode() default: changed from true to false (production-safe)
- Fix process.env -> env bindings in worker.ts requireAuth for Cloudflare Workers
- Fix Bun.file() crash in Workers: add try/catch with fallback
- Fix Bun.CryptoHasher -> Web Crypto API in mimo-auth.ts for Workers compat

Architecture:
- Add public methods to ProxyPool (getEntryAtIndex, getProxyUrlAtIndex, getCurrentIndex, setCurrentIndex) to remove all 'as any' casts in SessionProxyPool
- Add addProxy() method for manual proxy management
- Add loadAsync(), tryLoadAsync(), loadFromString() to ProxyPool

Serverless Stability:
- Add optional DNS rebinding protection via SSRF_DNS_CHECK env flag
- CORS cache now auto-invalidates when CORS_ORIGIN env changes
- Rate limiter max-size eviction (10k keys) prevents unbounded memory growth

Tests:
- Fix type assertions in test files (body as Record<string, unknown>)
- All 153 tests pass, typecheck clean

api/relay.ts

@@ -252,7 +252,7 @@ async function handleRelay(req: Request): Promise<Response> {
 	}
 
 	// ── Middleware: Rate limiting ────────────────────────────────────
-	const rateCheck = rateLimiter.check(clientIP);
+	const rateCheck = await rateLimiter.checkAsync(clientIP);
 	if (!rateCheck.allowed) {
 		logRelayEvent({
 			method,