feat(subsonic): wire /rest/scrobble into playevents.Writer

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.

internal/subsonic/stream_test.go

@@ -4,8 +4,6 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
-
-	"github.com/jackc/pgx/v5/pgtype"
 )
 
 func TestFindSidecarCover(t *testing.T) {
@@ -53,26 +51,3 @@ func TestImageContentType(t *testing.T) {
 		}
 	}
 }
-
-func TestNowPlayingMap(t *testing.T) {
-	m := newNowPlayingMap()
-	user := "user-1"
-	if _, ok := m.Get(user); ok {
-		t.Fatalf("unexpected entry on fresh map")
-	}
-
-	var trackID pgtype.UUID
-	_ = trackID.Scan("6ba7b810-9dad-11d1-80b4-00c04fd430c8")
-	m.Set(user, trackID)
-
-	entry, ok := m.Get(user)
-	if !ok {
-		t.Fatalf("Set did not persist")
-	}
-	if uuidToID(entry.TrackID) != "6ba7b810-9dad-11d1-80b4-00c04fd430c8" {
-		t.Errorf("trackID round-trip broke: %q", uuidToID(entry.TrackID))
-	}
-	if entry.At.IsZero() {
-		t.Errorf("At timestamp not set")
-	}
-}