Validates seed_track + optional limit (default cfg.Recommendation.RadioSize,
clamped to RadioSizeMax). Calls recommendation.LoadCandidates +
recommendation.Shuffle. Prepends seed to result. Server seeds a
math/rand source at startup; handlers package threads that as a
func() float64 so tests inject deterministic RNGs.
Mount + server.New gain a RecommendationConfig parameter.
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.
Discriminated-union JSON over three event types: play_started,
play_ended, play_skipped. Auth via existing RequireUser. play_started
returns play_event_id + session_id; the other two return { ok: true }.
Validates ownership: play_ended/play_skipped on another user's row
returns 403. Mount signature now takes EventsConfig and constructs
the playevents.Writer; server.New propagates it through.
bvandeusen