One cache instance per fire-reminder run. Each unique mediaId gets
prepared (uploaded to WA CDN) exactly once, and subsequent group
sends within the run reuse the prepared message via relayMessage.
Concurrent gets coalesce into a single prepare. Failed prepares
don't poison the cache — next caller retries.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>