Add an in-memory resourceId->local-path cache (24h TTL, 256-entry LRU) to
MediaResolveMiddleware so the same Yuanbao resource isn't re-downloaded when
it's referenced more than once in a session (own attachment, then quoted, then
group-observed backfill). Each reference otherwise triggers a fresh token
exchange + COS download.
The cache verifies the file still exists on disk before returning a hit (cache
dir may be swept) and is threaded through all three resolve paths:
_resolve_media_urls (rid parsed from placeholder URL), _collect_observed_media,
and the DispatchMiddleware quote path.
Salvaged from PR #30418 by @loongfay; the broader middleware refactor in that
PR converged with work already merged on main, so only the net-new download
cache is carried over.
f247686c42