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/queries/events.sql

@@ -0,0 +1,50 @@
+-- name: GetMostRecentPlaySessionForUser :one
+SELECT * FROM play_sessions
+WHERE user_id = $1
+ORDER BY last_event_at DESC
+LIMIT 1;
+
+-- name: InsertPlaySession :one
+INSERT INTO play_sessions (user_id, started_at, last_event_at, client_id)
+VALUES ($1, $2, $2, $3)
+RETURNING *;
+
+-- name: TouchPlaySessionLastEvent :exec
+UPDATE play_sessions
+SET last_event_at = $2,
+    track_count = track_count + 1
+WHERE id = $1;
+
+-- name: GetOpenPlayEventForUser :one
+-- Returns the most recent play_event for a user where ended_at IS NULL.
+-- Used by the auto-close-prior step in playevents.RecordPlayStarted.
+SELECT * FROM play_events
+WHERE user_id = $1 AND ended_at IS NULL
+ORDER BY started_at DESC
+LIMIT 1;
+
+-- name: InsertPlayEvent :one
+INSERT INTO play_events (
+    user_id, track_id, session_id, started_at, client_id
+) VALUES ($1, $2, $3, $4, $5)
+RETURNING *;
+
+-- name: UpdatePlayEventEnded :one
+-- Closes a play_event by id with the given ended_at, duration, and skip flag.
+-- completion_ratio is computed from duration_played_ms and the track duration
+-- (looked up by the caller — sqlc doesn't do joins on UPDATE).
+UPDATE play_events
+SET ended_at = $2,
+    duration_played_ms = $3,
+    completion_ratio = $4,
+    was_skipped = $5
+WHERE id = $1
+RETURNING *;
+
+-- name: GetPlayEventByID :one
+SELECT * FROM play_events WHERE id = $1;
+
+-- name: InsertSkipEvent :one
+INSERT INTO skip_events (user_id, track_id, session_id, skipped_at, position_ms)
+VALUES ($1, $2, $3, $4, $5)
+RETURNING *;