5001: attachment source capacity is busy; retry shortly

attachment source capacity is busy; retry shortly. This is an HTTP 503: a dependency is unavailable right now.

• Wait the number of seconds in the Retry-After header (and retry_after in the body), then retry the same request unchanged.
• Nothing about the request is wrong: this is backpressure, so re-sending it after the wait is the correct handling.
• Spread sustained load rather than retrying in a tight loop; the window is per brand.