ListRecentSessionTracks + UpdatePlayEventVector for the vector capture
path inside RecordPlayStarted. LikeTrack switches to :execrows so the
contextual-likes capture can detect insert vs already-exists. Existing
callers updated to ignore the count for now; later tasks consume it.
Tables and indexes per spec §5. session_vector_at_play ships nullable
so M3 doesn't need a follow-up migration. Table is named play_sessions
to avoid collision with the existing sessions (HTTP auth) table from
migration 0004.
bvandeusen