Get one archive's message bodies on demand, scoped to the project's chat.

The archive UUID doubles as a strong ETag (content never changes once written), so a matching If-None-Match short-circuits to 304 before the expensive blob read. The validator is client-constructible from the URL, so the short-circuit is gated on a cheap existence+scope check — a forged validator for an out-of-scope or missing archive still 404s. Blob validation is intentionally skipped on the 304 path: blobs are immutable after write, so a validator for a corrupt archive cannot come from a real 200, and a fresh (unconditional) read still 422s.