feat(db): add M2 schema — play_sessions, play_events, skip_events

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.

internal/db/migrations/0005_events.down.sql

@@ -0,0 +1,3 @@
+DROP TABLE IF EXISTS skip_events;
+DROP TABLE IF EXISTS play_events;
+DROP TABLE IF EXISTS play_sessions;