Initialize media estimates outside the global cache lock

[copyberry]

Initialize media estimates outside the global cache lock

Why

Image and audio estimate cache misses computed values while holding the cache mutex, blocking access to unrelated keys during initialization.

What changed

Add BlockingLruCache::get_or_init using Arc> and migrate the image and audio estimate caches to it. Initialization and same-key waits run in a Tokio blocking region after releasing the cache mutex. Callers share initialization while an entry remains cached; eviction can allow another initialization, so factories must be deterministic.

Testing

Add a concurrency test covering access to other keys during initialization, eviction of an in-flight entry, and successful completion of both the evicted initializer and its replacement.