Capability detection is already name-based (microsecond-fast), so caching it
wouldn't help — the real cost is huggingface_hub.scan_cache_dir() + the per-file
size sums walking the whole cache. So instead of re-walking on a TTL, the
cached-models/cache-stats layer now computes a cheap change-signature (the names +
mtimes of the top-level model dirs/files — a handful of stat() calls) and only does
the expensive scan when that signature changes. Unchanged caches return the last
result instantly and never re-walk; a change refreshes in the background. A long
600s backstop TTL still re-scans occasionally. Mutating ops reset the signature.

Net: the models page is instant on every load and the background refresh only runs
when models are actually added/removed/downloaded.

Verified: unchanged signature → 0 rescans; changed signature → one background rescan.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011DDv7BchtZQWsnPG6Jm49m