Resolve Chat Image Urls

Batch variant of /chat-image-url: mint signed URLs for many private chat-image gs:// refs in ONE round-trip. The message list resolves all visible images at once (was one POST per image, each a separate GCS existence round-trip + auth). The per-object .exists() checks run concurrently in a thread pool so the batch is bounded by the slowest single check, not their sum.

Returns {results: {ref: {url} | {expired: true} | {error: true}}}. Same bucket restriction and per-object expiry semantics as the singular endpoint.