Cache validation doesn't detect new library items on launch

[mmcdole]

Added new movies to my library, but on launch Kino checked the library updated timestamp and didn't detect an update - the new movies weren't shown until manual refresh.

Current behavior:

• On startup, SyncLibrary() calls IsValid(libID, lib.UpdatedAt)
• Compares stored cache timestamp against server's library UpdatedAt
• If storedTS >= serverTS, cache is considered valid

Suspected issue:
The server's library UpdatedAt timestamp may not always update when new items are added (behavior varies by server type / scan method). This causes stale cache to be served.

Possible solutions:

• Also compare item counts (if available from API)
• Verify what UpdatedAt actually represents for Plex/Jellyfin