Add Jellyfin NFO sidecar generation for downloaded videos

Writes a Kodi/Jellyfin-compatible .nfo XML file next to each .mp4 on
download completion, deletes it when the download record is removed, and
exposes POST /api/downloads/nfo/generate to backfill NFOs for existing
downloads. Frontend adds a "Generate NFO" button in the Downloads header.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/index.js

@@ -167,3 +167,4 @@ export const fetchChannelPlaylists = (channelId) => api.post(`/playlists/channel
 export const getPlaylistVideos = (playlistId, offset = 0, limit = 60) =>
   api.get(`/playlists/${playlistId}/videos`, { params: { offset, limit } });
 export const indexPlaylist = (playlistId) => api.post(`/playlists/${playlistId}/index`);
+export const generateNfoFiles = () => api.post("/downloads/nfo/generate");