Return 429 for sharing/upload-link rate limits instead of 500

Commit notes

The sliding-window helpers in share-links and upload-links threw plain Error on breach, which the global handler maps to 500 with the message suppressed in production. They now throw RateLimitError with the limiter's resetIn, so clients get 429 + Retry-After like every other rate-limited path.

[private session redacted]