Library scans and availavilit sync fetched the full /tv/:id payload including aggregate_credits and
cached it for 12 hours in the same unbounded cache the interactive TV detail page uses, so a full
scan left most of a library resident until the TTL expired. Scanners and availability sync now
request only keywords and external_ids, which is everything they read, and write to a separate
LRU-bounded cache with a 15 minute TTL, so scan lookups age out on their own instead of evicting
traffic of manual browsing. The interactive cache gains a key-count backstop against the unbounded
growth, and aggregate_credits.crew and credits.cast are stripped before caching since nothing in the
codebase readss that either.

fix #3307