Transcode state lives only in the process that started ffmpeg, so a pod restart
drops every in-flight HLS stream with no way for a peer to pick it up.
- add ITranscodeSessionStore plus the TranscodeSession and LiveStreamSession records
- add RedisTranscodeSessionStore with TTL leases and an atomic Lua takeover script
- add NullTranscodeSessionStore for single-instance deployments
- pick the store from Jellyfin:TranscodeStore:RedisConnectionString at startup
