Plexyfin 0.6.3.0

What's Changed

• Skip re-downloading artwork that hasn't changed in Plex (compares Plex updatedAt against the local image file), with a new Force Replace All Artwork override for one-time full refreshes
• New Max concurrent operations setting (default 3) caps simultaneous requests against Plex and Jellyfin during artwork sync, replacing previously unbounded parallelism for seasons/episodes
• Artwork is now processed in real batches with a configurable pause between batches
• Request external IDs (includeGuids) from Plex for more reliable item matching
• Enable nullable reference types and clean up build warnings

Thanks to @cHILLa-1 for the artwork sync improvements in #38!

Full Changelog: v0.6.2.0...v0.6.3.0