submission=false → play_started (replaces in-memory nowPlayingMap).
submission=true (default) → synthetic completed play. The nowPlayingMap
struct + factory + the nowPlaying field on mediaHandlers are deleted;
play_events WHERE ended_at IS NULL is now the source of truth for
'currently playing,' reachable from M3+ work.
api.Mount now takes the shared *playevents.Writer instead of cfg so
the same writer instance feeds both surfaces.
http.ServeContent handles Range/ETag/If-Modified-Since; download forces
attachment disposition. getCoverArt tries albums.cover_art_path, then
falls back to cover.{jpg,jpeg,png}/folder.{jpg,jpeg,png} next to the first
track's file. scrobble submission=false records a track into an in-memory
nowPlaying map keyed by user — M2 will read this and wire real events.
bvandeusen