feat(db): add contextual_likes table (M3 session-context capture)

Table was referenced in migration 0005's comment but never created —
this slice fills the gap. Soft-delete via deleted_at column; hot-path
partial index on active rows; GIN index on session_vector for
M3 sub-plan #3's similarity queries.

internal/db/queries/contextual_likes.sql

@@ -0,0 +1,10 @@
+-- name: InsertContextualLike :exec
+INSERT INTO contextual_likes (user_id, track_id, session_vector, session_id)
+VALUES ($1, $2, $3, $4);
+
+-- name: SoftDeleteContextualLikesForUserTrack :exec
+-- Marks all currently-active rows for (user, track) as deleted. Idempotent —
+-- already-deleted rows aren't re-touched.
+UPDATE contextual_likes
+SET deleted_at = now()
+WHERE user_id = $1 AND track_id = $2 AND deleted_at IS NULL;