Disable rate limiting on relay endpoint (#745)

## Summary
- Relay calls from the SDK don't attach `api_key` or `app_name`, so they
fall back to IP-based rate limiting at 5 RPS default
- Batch track uploads (e.g. 22 tracks via `Promise.all`) exceed this
limit, causing `EntityManagerClient.manageEntity` to fail with "API Rate
Limit Exceeded"
- Skips rate limiting on `/relay` paths until the SDK properly
identifies itself on relay calls

## Test plan
- [ ] Verify batch track upload (20+ tracks) no longer hits rate limit
errors
- [ ] Verify rate limiting still works on `/v1` API endpoints

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: raymondjacobson

api/rate_limit_middleware.go

@@ -109,6 +109,11 @@ type RateLimitMiddleware struct {
 // Middleware returns the Fiber handler. Pass apiServer to resolve identifier from Bearer or Basic Auth signer first; if nil or no signer, falls back to api_key/app_name query params.
 func (rlm *RateLimitMiddleware) Middleware(apiServer *ApiServer) fiber.Handler {
 	return func(c *fiber.Ctx) error {
+		// Skip rate limiting for relay endpoints
+		if strings.HasPrefix(c.Path(), "/relay") {
+			return c.Next()
+		}
+
 		var identifier string
 		if apiServer != nil {
 			signer, err := apiServer.getApiSigner(c)